The objective of the study is to find out: (1) how rectangular module with the STEM approach to improve valid and practical critical thinking skill; and (2) the effectiveness of the rectangular module development with the STEM approach to improve critical thinking skill. It is a research and development (R & D) study. This research consists of two phases, namely preliminary study and product development. Product development consists of product testing using one group pretest-posttest design experimental research. The population in this study is the students of one State Junior High School in the Kebumen region by taking five students as the sample for individual tests, 32 students for limited scale tests, and 64 students for wide-scale tests. The results of this study indicate that (1) the rectangular module with the STEM approach on the aspects of content, presentation, graphical, and language feasibility are excellent, the STEM approach feasibility and components of critical thinking skills are good category; and (2) the effectiveness shows that the rectangular module with the STEM approach to improve critical thinking skills is proven effective which can be seen from the average value of N-Gain in the experimental class of 0.37 with the category of medium improvement, while N-Gain in the control class is 0.03 with low increase category.