The employed research instrument was a critical thinking test comprising 48 items for Mattayom Suksa III students. Content validity of the developed test was verified by the item-objective congruency (IOC) index. Each item’ s difficulty level and discriminating power were verified with the difficulty index and discriminating index. Its construct validity was verified by confirmatory factor analysis (CFA). Its reliability was verified by calculation of Kuder- Richardson 20 (K-R 20) formula.