Several studies and related literature stated that in an effective teaching process, teachers need to have mathematical content knowledge and pedagogical knowledge to be able to teach mathematics deeply and broadly.In this study, researchers worked with pre-service primary teachers through teaching practice activities to see how mathematical content knowledge influenced their mathematical knowledge for teaching.Data collection was carried out through observation, interviews, and field notes.The collected data were analyzed by induction and deduction methods.The results of the study showed that when pre-service primary teachers were lacking in mastering mathematical content knowledge, it hindered pedagogical content knowledge and mathematical knowledge for teaching as well.Their weakness in mathematical content knowledge can be seen when they interpreted and evaluated students' explanations.They also found it difficult to provide feedback to students.For research subjects who had good mathematical content knowledge, but not optimal pedagogical content knowledge, learning was also not effective.They seemed to have difficulty bringing students to contextual situations. As a result, they used the knowledge that students have not understood yet.