Purpose: This study addresses the need for a validated tool to assess self-directed learning (SDL) behaviors among Thai operational employees in the petrochemical industry. Understanding SDL capabilities is essential for effective human resource development, given the complexity of plant operations and the need for continuous skill enhancement. Design/methodology/approach: This research adapts the self-rating scale of self-directed learning (SRSSDL) to the Thai workplace context. The SRSSDL was translated into Thai and adapted for the workplace. Five experts validated the content, and validity and reliability testing was conducted with 50 operational employees from Thai petrochemical plants. Confirmatory factor analysis (CFA) and Cronbach's alpha were used to assess the instrument's reliability and validity. Findings: Content validation confirmed that all SRSSDL items had an item-objective congruence (IOC) score above 0.5. CFA results showed a good model fit (comparative fit index [CFI] = 0.99, Tucker-Lewis index [TLI] = 0.98, root mean square error of approximation [RMSEA] = 0.02 and standardized root mean square residual [SRMR] = 0.02). Reliability analysis showed Cronbach's alpha values from 0.91 to 0.96, indicating excellent internal consistency. Originality/value: This research is the first study to assess and prove the content validity, construct validity and reliability of the SRSSDL in the context of Thai workplaces, particularly in the petrochemical industry. The SRSSDL is a reliable and valid tool for assessing SDL behaviors among Thai operational employees. It enhances human resource development by identifying SDL capabilities and fostering continuous improvement and SDL.