The purpose of this study is to determine the effect of an activity set developed according to the inquiry-based learning (IBL) approach in the unit "Particulate Structure of Matter" on students' critical-thinking skills in science and technology courses. The study was conducted with 90 students from the 6th grade attending four, 6th grade secondary school classes. Within the framework of the study, in order to evaluate the effects of IBL approach on the students' critical-thinking skills in science and technology courses, the guided activity set was developed by the researchers in line with the IBL approach. In this study, pretest and posttest control group experimental designs were applied. The findings of the study revealed that science and technology learning supported with the guided activities developed in line with the IBL approach have significant effects on students' critical-thinking skills in science and technology courses.