As various means of computer mediated communication (CMC) have been incorporated within language classrooms over the past two decades, it has become important to critically understand whether, to which extent, and under what contextual factors, a particular type of CMC is more effective than other modes of communication. This study examined the magnitude of the effect of text-based synchronous computer-mediated communication (SCMC) on second language acquisition (SLA). By searching the studies published between 1990 and 2012, this meta-analysis explored 10 experimental and quasi-experimental journal articles and doctoral dissertations and reports their overall effect on SLA, and the contextual factors that influence the between-study variation. A small but positive overall effect (m = 0.33) indicates that text-based SCMC could make a larger difference on SLA than other means of communication. Findings further suggested that intermediate learners may benefit more from SCMC tasks if they are grouped into pairs or small groups and participate in SCMC interactions on a weekly basis. In terms of suggestions for future research, authors should provide more description about the SCMC task in order to confirm or disconfirm the factors that are associated with effectiveness of second language (L2) learning in technology-mediated language learning contexts.