This study investigated student interaction patterns and their co-regulation practices in text-based and multimodal computer mediated collaborative (CMC) writing. To this end, 30 EFL (English as foreign language) participants' online collaborative writing performances were analyzed. The analysis included conversation analysis on the transcription of stored conversations of multimodal Moodle and discussion logs of online text-based writing Forum. Data were coded according to Storch (Storch, Language Learning 52:119-158, 2002)'s collaboration patterns coding scheme in order to trace interactional styles (collaborative pattern, dominant/passive pattern, dominant/dominant, and expert/novice pattern). Besides, data were coded to trace co-regulation patterns (planning, executing, monitoring, evaluation, orientation and elaboration). The chi-square analysis indicated that there were significantly more collaborative patterns in multimodal and expert/novice pattern in text-based CMCs. Co-regulation practices of "Elaboration" and "evaluation" occurred more in text-based CMC whereas "monitoring" practice had the highest occurrence in multimodal CMC. "Executing", "planning" and "orientation" indicated no significant difference of occurrence. The results also indicated that there were significant differences in interaction patterns in each co-regulation practice in text-based and multimodal CMCs. The results imply that both mediums showed benefits in preparing learners for learning process but facilitated learning in different ways and consequently, they prepared learners for distinct collaborative composing processes.