This study examined the effects of pretask training to promote peer collaboration, encourage learning opportunities, and foster individual L2 writing development in web-based L2 collaborative writing (CW) tasks. The participants were 48 students from two junior English composition classes at a Taiwanese university. One class (n = 24) was assigned to be a pretask training (PT) group and the other (n = 24) to a no pretask training (NPT) group. Both groups completed an individual pre- and post-test writing, and two L2 CW tasks via Google Docs. The PT group received pretask training before the CW tasks, whereas the NPT group did not. The interaction between the learners was analyzed for the number, outcome, and engagement level of content-, organization-, and language-related episodes (LREs) and for the learners' interaction patterns. Pre- and post-test writing was analyzed in terms of content and organization and language complexity and accuracy. The PT group: (a) produced more collaborative interaction during the CW processes than the NPT group, (b) produced more LREs and correctly resolved a greater proportion of LREs and content-related episodes, and (c) made greater improvement in content and language accuracy of individual L2 writing.