This research aims to investigate the effect of problem-based learning and problem-based activity design implementations on the conceptual learning of pre-service science teachers and to determine their views on the effects of the process. The method of research is the pre-test post-test quasi experimental design. Thirtyeight pre-service science teachers participated in the study. The conceptual understanding test and semistructured interview questions developed by the researchers were used as data collection tools in the study. Three separate learning modules, each consisting of two sessions on physics, chemistry, and biology, were prepared by the researchers for use in the experimental process, which lasted eight weeks. In the experimental group, the courses were carried out with problem-based learning modules containing science experiments in each session, and in the control group with only semi-open-ended science experiments. Besides, at the end of the sessions, the pre-service teachers in the experimental group designed the problem-based science experiments for secondary school students. In contrast, the pre-service teachers in the control group designed only science experiments for secondary school students. Non-parametric tests were used in the analysis of the data as the distribution of the data was not normal. As a result of the analysis of the data, it was determined that at the end of the experimental implementation process, the conceptual understanding levels of the preservice teachers in the experimental group were significantly higher than the pre-service teachers in the control group. In the interviews, pre-service science teachers stated that the problem-based learning method contributes to their learning, increase their attention to the course. However, they emphasized that they have difficulty in solving problems, doing research, and revealing their existing knowledge.