This article reflects a special part of a research conducted to examine the effect of lesson study on prospective classroom teachers' mathematical pedagogical content knowledge (MPCK). In this article, the special part consists of prospective teachers' knowledge of lesson planning including a mastery of planning an affective lesson taking into account student's current knowledge, understanding and difficulties within mathematics. Therefore, the research question is how lesson study practices affect prospective classroom teachers' knowledge of lesson planning as a sub component of MPCK. The research is conducted with 12 prospective classroom teachers, six of them have already assisted to lesson study and the others have not. Data collection tools consist of video records, class observations, field notes, interviews and lesson plans prepared and used by prospective teachers participated in lesson study. Findings indicated that the prospective classroom teachers who participated in lesson study improved their knowledge in terms of planning an affective lesson taking student's current knowledge and understanding into consideration. They appeared to be aware of selecting and ordering appropriate activities related to the actual objectives of the mathematical topics. They also appeared to be better in lesson organization and lesson presentation comparing to the other group of prospective teachers who did not participated in lesson study.