This study proposes a Wiki-based collaborative writing approach to the writing process for EFL (English as a foreign language) learners. A five-stage computer-mediated collaborative writing project including collaborative planning, partitioned drafting, peer-revising, peer-editing, and individual publishing was blended with on-campus English composition course. Fifty-one L2 learners at a university in central Taiwan participated in this project. Procedural scaffolding and collective scaffolding were provided to promote students' self-regulation and thus to foster the development of students' writing skills. A cross-referencing questionnaire survey was adopted to investigate students' perceptions of Wiki-based collaborative writing and students' perceptions of their work in each stage of collaborative writing. As the results indicated, a high percentage of students' satisfaction showed positive perceptions of this Wiki-based collaborative writing environment, and the instructional design of implementing a Wiki-based collaborative writing project with a five-stage writing process does assist EFL learners to accomplish a collaborative writing task on the internet with less limitation of time. This article also points to new possibilities for future research. (Contains 1 figure.)