Abstract-This study aimed to develop and validate theCase-based Learning module to improve the critical thinking skills of high school students. The development model used was the 4-D model that includes, define, design, develope, and disseminate. Module validation was conducted through validity test, practicability test, and effectiveness test which revealed that the module developed has a very high validity with an average score of 4.25 with a very high practicality with an average score of 4.21, and it has the effectiveness to improve critical thinking skills with an average value of the critical thinking skill test of 85.92. Result of t test get t value 22,567> t table (1,868) which means that there is a significant improvement of critical thinking skill of students after the implementation of the module. Thus, the module developed is valid, practical, and effective to improve critical thinking skill of high school student.