This study aims to obtain an overview of improving students' mastery of science concepts through the application of guided inquiry learning models on light and optical materials. The research was conducted using a quasi-experimental method with the nonequivalent control design. The population of this study was all students of class VIII at one MTs in Pekanbaru city with a sample of two classes with a sampling technique in the form of purposive sampling. Data collection was carried out using a test instrument for mastery of science concepts on light and optical materials. Hypothesis testing was carried out using the independent sample t test. The results showed that the increase in students' mastery of science concepts in the experimental class was better than the control class where the normalized gain average score <g> of students in the experimental class was 76.12 in the high category while the control class was 60.01 in the medium category. Based on the hypothesis test, it was concluded that there was a significant difference in students' increasing mastery of science concepts between the experimental class that applied the guided inquiry learning model and the control class that applied the conventional learning model on light and optical materials.