The aim of this study was to determine the effect of inquiry-based laboratory activities on the students' understanding of the nature of science (NOS) in learning lights and optics topic. The method used in this research is quasi experiment. Sampling technique using random sampling to class and the samples were taken from grade 8 in one of junior high school in Bandung. The sample was 45 students, consisting of the experimental class (n = 24 students) and the control class (n=21 students). The experimental class is taught by inquiry-based laboratory activities, while the control class is taught by non-inquiry laboratory activities. The result of this research shows that the experimental class got N-Gain of 0.60, while the control class got N-Gain of 0.44, and both classes proven to have statistically significant different improvement.