Pedagogical Content Knowledge (PCK) is a fundamental component for effective teaching and plays a crucial role in teacher education. Even though pre-service teachers are engaged in training to improve their PCK, the literature reveals that most of their difficulties in practicum courses stem from their PCK. The purpose of this research is to describe the effects of the Lesson Study on Pre-service Primary Teachers' (PPTs) mathematical PCK in terms of the following components: "students' knowledge, lesson preparation" and "delivery." For this purpose, we observed and evaluated 12 PPTs' lessons during the Teaching Practicum-II course. We assigned six PPTs to the lesson study group and six of them to the non-lesson study group. Each PPT from the first group had already completed three different Lesson Study cycles during the Teaching Practicum-I course. The data consisted of the video recordings of classroom teaching, classroom observations, field notes and one-on-one interviews with PPTs. We analysed the data using qualitative analysis techniques. The results revealed that, although there were some differences among PPTs concerning the aforementioned components, Lesson Study helped PPTs improve their teaching practice and their PCK.