The purpose of this research is to find out the practicality and effectiveness of student worksheet using IBL assisted by basic mathematics skills to develop critical thinking skills. The method used was quasi-experimental designs with pretest-posttest type. The practicality was obtained through the results of observations of the learning implementation and student responses questionnaire. The effectiveness was obtained through pretest-posttest data analyzed by N- gain and effect size. The results of the observation of the learning implementation and student responses questionnaire were included in very good and very high categories. N-gain test in control and experiment groups was considered in low and medium categories. While, the effect size obtained was in very strong category. In general, the result shows that the student worksheet has met the criteria of practicality and effectiveness.

Keywords : Student worksheet, IBL, Mathematic Skills, Critical Thinking Skills.