The aim of this study was to investigate pre-service primary school teachers' (PPSTs) pedagogical content knowledge (PCK) on quadrilaterals. In this study, the PCK components of knowledge of understanding students (KUS) and knowledge of instructional strategies (KIS) were used. The participants of the study consisted of 83 PPSTs studying at the primary education department of a university in Turkey. The illustrative case study method was used, while six scenarios were used as the data collection tool developed by researchers. The data obtained from open-ended scenarios were analyzed by using the summative content analysis technique. As a result of the study, it was observed that the KUS of the PPSTs about quadrilaterals was not on the desired level. Moreover, the KIS of the PPSTs was also not on the desired level. As a result of the study, it was observed that, in the process of eliminating the mistakes of students, the PPSTs preferred mainly the "Direct Instructional" method, which is based on traditional approaches and centers the teacher.