This research aims to produce an assessment instrument of STEM-based critical thinking skill which meets the feasibility criteria. This development research refers to the model developed by Borg & Gall and is modified using the development model instruments developed by Oriondo & Antonio. The research subjects were one senior high school Physics teacher, 129 tenth grade students, and 331 eleventh grade students of senior high school. The data gathering was carried out using self-evaluation sheets, observation rubric, students' worksheets (LKPD) and report-scoring rubrics, and a test instrument on critical thinking skill. The research results show that the developed performance assessment has fulfilled the content validity based on the evaluation by 3 experts and 3 practitioners. The reliability of all the rubrics in the performance assessment is categorized as very high. The Test on critical thinking skill consisting of 72 items was declared fit usingPCM and the level of difficulty of the items ranged between -0.69 and 1.14, which implies good category. The test also had a reliability coefficient of 0.81 and was categorized as very high and suitable to measure the students whose ability ranged from -1.60 to 1.70 in the logit scale. In addition, teachers and students gave positive responses to the application of the developed assessment. Therefore, the developed performance assessment of STEM-based critical thinking skill has fulfilled the feasibility criteria to be applied in the senior high school physics class.