This study aims to examine how learners engaged in wiki-based collaborative writing when they were using their first language (L1) or second language (L2). Issues concerning similarities and differences in wiki collaborative writing activities, wiki participation levels, wiki interaction patterns and wiki collaboration levels between the L1 and L2 writing groups are discussed. This paper reports a case study of a group of Hong Kong secondary school students who were required to use “Google Sites” to complete their Liberal Studies group projects. Student’s wiki writings and comments in their L1 (Chinese) and L2 (English) were collected and examined. Results indicated that, while similarities in writing activity patterns, participation levels and collaboration levels were shown in the two groups, differences did exist in their interaction patterns, which were affected by the different ways they used the wiki comment sections. This study can help educators to become aware of the different needs of L1 and L2 groups and to implement wiki collaborative writing more effectively to support students.