The research aimed to describe of science process skills students and to assess the effect of guided inquiry learning to physics critical thinking skills students. The research used true experimental with randomized control group only design. Data collection methods used in this research is test, observation, documentation, and interview as supporting data. The data of student's critical thinking skills were analized by using Independent Sample T-Test in SPSS 22. The results showed 1) guided inquiry learning can be used to train students' science process skills. The term proved by percentage increase in science process skills of students from the first meeting until third meeting and if the categorized in science process skills criteria already quite good, 2) guided inquiry learning significant effect on students'critical thinking skills. It can seen from the significant value (one-tailed) of 0,0015 or ≤ 0,05. The research can be concluded that: the students' science process skills during guided inquiry learning was classified into good criteria and the student's critical thinking skills after learning physics using guided inquiry model better than the student's critical thinking skills after learning physics using the usual model in Senior High School.