The related literature emphasizes that mathematics content knowledge (MCK) itself is not sufficient for effective teaching; teachers also need to have deep and wide mathematics pedagogical content knowledge (MPCK). In this study, we worked with 12 pre-service primary teachers through Teaching Practice course in order to examine how their MCK affects their MPCK. Observation, interview and field notes were used as data collecting tools and data gathered were analyzed with induction and deduction methods. The results showed that pre-service teachers' lack of MCK made it difficult for them to use MPCK effectively. Their weaknesses in MCK were apparent in their evaluation and interpretation of the students' responses and explanations, as well as in giving feedback to these comments. On the other hand, in cases where pre-service teachers' MCK was good, but their MPCK was insufficient, the lessons were also ineffective. In these instances, they had difficulties in reducing the level of instruction to the level of the students, and they used some knowledge which had not been mastered by the students.