Short Answer
An employer may be able to extend the assessment period by agreement or under a properly drafted policy, but extending probation does not normally restart the employee’s service clock.
For Thai labour-law purposes, probationary employment is still employment. The employee’s continuous service runs from the original employment start date.
That matters because statutory rights can arise by reference to actual service. In particular, section 118 of the Labour Protection Act gives a dismissed employee statutory severance once continuous service reaches 120 days, unless a statutory exception applies.
The familiar practice of setting probation at 119 days is therefore a risk-management convention, not a general legal rule that 120 days is the maximum lawful probation period.
Why This Matters
HR often treats probation as a separate preliminary stage:
“The employee has not passed probation yet, so their normal employment rights have not started.”
That is too broad.
Thai law does not place a probationary employee outside the employment relationship.
The source matter behind this note illustrates the problem well. The draft policy allowed probation to be extended or suspended for leave and performance reasons and also contained automatic-confirmation mechanics. A reviewer comment treated a customary probation period as a legal maximum, but the source register correctly preserved that statement for research rather than repeating it as law. fileciteturn15file7L202-L210
Typical Scenario
An employee is hired subject to a probationary assessment.
Near the planned end date, the employee has taken substantial leave or management wants more time to assess performance.
HR proposes extending probation by another month.
Management asks:
“If we extend probation, does the 120-day employment threshold move as well?”
No. The assessment timetable and continuous service are different concepts.
Practical Analysis
01Treat probation as part of continuous employment.
Thai case law has treated probationary employment as employment under an indefinite-term relationship where continued employment depends on the assessment outcome.
The service period therefore runs from the employee’s original start date.
Calling a later period “extended probation” does not create a new hire date.
02Understand what the 120-day point actually does.
Section 118 provides the first statutory severance tier once an employee has worked continuously for 120 days and is dismissed without falling within an applicable no-severance exception.
That is why many employers historically design probation to finish before day 120.
The statute does not say that probation itself becomes unlawful on day 120.
03Extending the assessment can change the termination cost.
If the employee crosses the 120-day threshold during extended probation and is then dismissed for ordinary performance reasons, statutory severance may become payable.
Notice or pay in lieu of notice must also be analysed separately.
A “failed probation” label is not itself a section 119 misconduct ground.
04Check the contract and policy before extending.
Does the employment contract permit an extension? Does the work rule explain when it can occur? Is employee agreement required? Is there an automatic-confirmation clause when no decision is issued by a particular date?
An extension contrary to the employer’s own documents can create a separate dispute even if the concept of extended assessment is not prohibited.
05Be cautious about suspending the service clock for leave.
A policy can say that management needs more active working time to assess performance after a long absence.
That does not mean statutory continuous service automatically stops running during the leave.
The assessment calendar and the legal service period should be recorded separately.
06Use performance evidence, not probation status alone.
If the issue is poor performance, document objectives, feedback, errors, support and the reason for further assessment.
Extending probation indefinitely without a clear purpose can make the process look arbitrary.
What We Usually Look At
We review the employment start date, planned probation end date, contract, work rules, extension clause, leave taken, assessment records, payroll cycle, notice requirements, 120-day severance threshold, termination reason and any automatic-confirmation wording.
Common Traps
Typical mistakes are treating 120 days as a statutory maximum probation period, restarting service when probation is extended, assuming failure of probation removes severance or notice obligations and suspending the service clock because the employee took leave.
Practical Next Step
Use two timelines:
assessment timeline: start → reviews → leave / interruption → extension → final decision
legal service timeline: original start date → 120 days → later service thresholds → termination date
Never let the first timeline overwrite the second.
Legal Sources
- Labour Protection Act B.E. 2541 (1998), sections 17, 118 and 119.
- Supreme Court Judgment No. 5131/2550, holding that probation does not exclude notice or severance rules and that service is counted from the original employment start date.
- Supreme Court Judgment No. 5249/2545, treating a probation arrangement as part of an indefinite-term employment relationship and applying statutory notice/severance principles.
General legal information only. Please seek advice on your specific facts. Laws and regulatory practice may change.