Primary school students frequently encounter plane figures' perimeter and area. Using an onto-semiotic approach, this study examine students' learning challenges when learning the plane figures' perimeter and area. As part of the didactic design research, this study used an interpretive paradigm. The study included 25 fourth-grade students in one of elementary school in Sidoarjo, Indonesia. Data was gathered through tests, interviews, and documentation, as well as descriptive analysis techniques. The results showed that students face three types of learning obstacles, i.e. ontogenic, epistemological, and didactic. Students have difficulty describing plane figures, understanding mathematical situations in problems, and using mathematical procedures to solve problems using the onto-semiotic approach. This study examines students' reactions to learning about the plane figures' perimeter and area. Furthermore, the findings of this study serve as a starting point for educators, practitioners, and researchers in developing new didactic designs. So that grade four primary students can overcome the obstacles discovered.