In this study, we developed a test tool to measure the computational thinking ability of middle school students and investigated their computational thinking power using the tool. The test tool used exploratory factor analysis to examine the computational thinking scales of Korkmaz et al. (2017) and derive suitable factors and questions for middle school students in Korea. The developed test tool was applied to 492 middle school students to analyze differences in computational thinking ability according to gender, grade, programming experience, type of programming language, and interest. According to the study, male Korean middle school students had higher computing power than females. In addition, students who had programming experience or used text-based rather than block-based programming languages demonstrated higher computational thinking. There was no significant difference in the computational thinking of middle school students according to grade, and the level of interest in artificial intelligence only had a slight influence on computational thinking.