Critical thinking is one of the most important thinking skills for students in solving mathematical problems. Hence, students will be able to compete on a global scale when they have the thinking skills; one of them is critical thinking skill. In solving the series of generalization problem, the researcherss divided the problem solving process into three levels of process solutions made by them. This descriptive research employed qualitative approach. The results of students’ work were analyzed to determine the level of students’ critical thinking skills. Based on the findings, students who had high mathematics skills were classified at critical level (TKBK 3). It indicated that the students were able to find generalization of series. On the other hand, students who had medium mathematics skills were classified at the critical level (TKBK 1). It indicated that they were only able to find the formula Sn to each series, whereas low skills students were at the uncritical level (TKBK 0). It indicated that they were only able to sort out what was known and asked questions.