A growing number of researchers have acknowledged the potential for using wikis in online collaborative language learning. While researchers appreciate the wikis platform for engaging students in virtual team work and authentic language learning, many also have recognized the limitations of using wikis to promote student collaboration (Alyousef & Picard, 2011; Arnold, Ducate & Kost, 2009; Coniam & Kit, 2008; Judd, Kennedy & Cropper, 2010; Warschauer, 2010). The current study aims to examine what factors facilitated or hindered student collaboration when a wiki environment was used to engage 103 Taiwanese students from two universities in an online picture book production project. Divided into 17 groups of four to six members, the students spent approximately one academic year forming online communities, learning to conduct peer editing, and collaboratively completing a final learning product, an online picture book. A variety of data, including the electronically archived versions of the wiki pages, students' responses to retrospective surveys, and focused follow-up interviews were collected and analysed. The findings suggested that the nature of the learning tasks, students' constant communication and appreciation of different opinions, the difficulties they encountered when communicating asynchronously, and students' expectations toward English learning affected to what extent they were involved in the online collaboration.