摘要:The aim of this research was to produce a web-based instrument test to assess critical thinking skills that are feasible (valid, practical, and effective), and to describe the profile of students critical thinking skills. This research using Research and Decvelopment (R&D) model from Borg and Gall which was modified by Sukmadinata (2010) into three main steps : preliminary studies, development of assess, and validation of assessment instrument. Respondent of this research is 36 XI MIPA-3’s students in SMAN 1 Taman Sidoarjo. The web-based instrument test which developed has validited by media and istrument lecturers, practicality is measured using students and teachers response, and the effectiveness of the product is seen through the measurement results of critical thinking skills using the developed web-based instrument test. Based on the research, obtain the results: 1) The web-based instrument test are feasible with the media and material validity in the very high category with a percentage 90% and 92,5%, the product is very practical based on measurements from users with a percentage 91,4%, the effectivity of product which developed indicated by the measurement of each student’s critical thinking skills. 2) The profile of students critical thinking skills in XI MIPA-3 is known that 3 student have critical thinking skills in the very high category, 11 students in the high category, 20 students in the medium category, and 2 students in the low category. The indicator of critical thinking skills that most student master is interpretation, and the less mastered indicator is evaluation.