The aim of the study is to identify and explain preparation activities of three English teachers in lesson study process. The English teachers dealt with activities such as setting objectives, building content, organizing classroom practices and identifying success criteria for learning objectives and reflection in three different lesson study cycles based on social constructivism for about 5 months. In this study, the single case study design was adopted. Criterion sampling method was used to determine the participants. The group-based activities of teachers were repeated in each research lesson at different subjects and different grades. Instead of preparing a new format of the existing lesson plan in each research lesson, the participants prepared new lesson plans for each research lesson. The data were obtained by class observation, camera recordings, interview, and document examination of the lesson plans. It was observed that the teachers made more qualified decisions in planning teaching tasks and there were positive changes in their planning skills and teaching practices in the process in lesson study cycle.