This study investigated how preservice teachers develop collective efficacy when preparing lesson plans for a science methods course, and how this collective efficacy changed over time. The data were collected from four preservice science teachers working in a group to fulfil the requirements of the course. Findings revealed that four factors, including collaborative work, sharing the same goal, attitudes towards group work and group cohesion, played an important role in the development of collective efficacy among preservice teachers. The study also showed that collective efficacy improved continuously over the semester.