Guided Inquiry Model is a learning model that stimulates students to find problems or questions, where the topic or learning material has been determined by the teacher. This study aims to analyze the effectiveness of guided inquiry model to improve the science process skills of high school students. This research is pre-experiment with research design using pre-test and post-test design that is tested to 30 high school students, which is divided into two groups. Prior to the learning with the guided inquiry model the students were given a test of science process skill and after the student's learning was re-given the same test. Data were analyzed using paired t-test and n-gain. The results showed that: there was a significant improve the science process skills in terms of a = 5%, the average of n-gain was 0.72% and was not different in both groups. Thus, the guided inquiry model can effectively improve the science process skills of High School students.