Computational thinking has received tremendous attention from computer science educators and educational researchers in the last decade. However, most prior literature defines computational thinking as thinking outcomes rather than thinking processes. Based on Selby and Woodland's framework, this study developed and validated the Computational Thinking Scale (CTS) to assess all students' thought processes of computational thinking for both general and specific problem-solving contexts in five dimensions: abstraction, decomposition, algorithmic thinking, evaluation and generalization. A survey including 25 candidate items for CTS as well as demographic variables was administered to 388 junior high school students in Taiwan. An explorative factor analysis using the principal axis method with the oblimin rotation was used to validate the scale. Finally, 19 items were extracted successfully under the designed five dimensions, with a total explained variance of 64.03% and an overall reliability of 0.91. Results of the demographic comparisons showed that boys had a greater disposition than girls in decomposition thinking when solving problems using computer programming. In addition, programming learning experience, especially self-directed learning and after-school learning, had significant positive effects on all dimensions of CTS. Several future studies are suggested using this tool.