In this study, collaborative lesson planning (CLP) was a positive 'dissonance' to the teachers' individual lesson planning practices. Thirty teachers grouped into grade-level teams (five members) participated in the study. Data transcripts from audio- and video-recordings, formal and informal interviews, and written evaluation forms were qualitatively analyzed through the constant comparison method. Results showed that through reflections-on-action, CLP had positive impacts on the teachers' instructional practices including the provision for: (1) continuous learning, (2) improvement of professional practice, and (3) reflective, affective, and professional support. Two stages of CLP were identified: the 'dissonance' stage and the refinement stage. The dissonance stage has three activities: (1) goal setting, (2) lesson implementation and observation, and (3) collaborative reflection and discussion. The refinement stage has four activities: (1) organization and reflection of prior knowledge, (2) reflection, organization, and development of new knowledge, (3) enhancement and enactment of the new knowledge, and (4) task accomplishment.