The goal of this research was to help improve vocational school quality by measuring teachers’ performance through internal supervision. Thailand is committed to using education as a tool for development. Therefore, education must be of a high quality that is influenced by
internal supervision by school administrators. The researcher wanted to study the effect of internal supervision on teachers’ performance in vocational schools. The purposes of this research were to study internal supervision and teachers’ performance, to study the relationship between
internal supervision and teachers’ performance and to create a prediction equation of internal supervision affecting teachers’ performance. The sample of this research was a group of 210 teachers in schools under the Office of the Vocational Educational Commission in Rayong, Chanthaburi and Trat Provinces in the 2017 academic year. The sample was selected by stratified random sampling. The research instruments used was a 2-part, five-rating scale questionnaire .Part 1 had 45 internal supervision items, and part 2 had 54 teachers’ performance items. The alpha-coeflicient levels of the two parts were .96 and 97. The statistics used for data analysis and hypothesis testing were: mean, standard deviation. Pearson-Product Moment Correlation Coeffcient and simple regression analysis.