This study investigates how Computer-Mediated Communication (CMC) can be effectively used in a teaching practicum course to enhance preservice teachers' learning. We constructed a web-based CMC system and used it in a Teaching Practicum course. Computer science preservice teachers and experienced secondary school computer teachers, who served as mentors, participated in the study. The research findings indicate that the preservice teachers considered the CMC system helpful for supporting their learning. The findings also indicate that some initiating/follow-up discussion styles of the mentors were more effective than others in promoting interactions in the discussion forum. It was also found that a teaching-video-enhanced CMC environment is valuable in improving quality of mentoring and in increasing preservice teachers' reflection on teaching.