The purpose of this study is to examine preschool preservice teachers' subject matter knowledge (SMK) and pedagogical content knowledge (PCK) of basic geometric shapes. The study employed case study method in order to investigate preschool preservice teachers' SMK and PCK on geometric shapes in actual classroom environment and to describe the situation in-depth. The study group was composed of two senior year students studying at the department of preschool teaching at a state university. Triangulation was employed in order to improve the validity, and the data of the study were obtained from semi-structured interviews, lesson plans prepared by the prospective teachers, and video recordings of teaching practice lessons. The results of the study showed that preservice teachers had some problems when describing geometric shapes and they did not use the mathematical language effectively. One of the preservice teachers was observed to have combined the new information with the children's prior learning. Both preservice teachers tried to teach geometric shapes by referring to daily life in order for students to understand the shapes better, and one of the preservice teachers also employed activities involving visual aids. The participants were found not to be fully informed about the possible student's misconceptions. One of the preservice teachers mentioned alternative evaluation methods for evaluating students' learning while the other preservice teacher did not mention these methods. Based on the results, it is suggested that when preservice teachers are being taught they should acquire SMK and PCK more effectively, and they should be provided with environment where they can implement what they learn. (Contains 3 tables and 8 figures.)