This paper reports on a study investigating the potential effect of synchronous and asynchronous text-based computer-mediated communication (CMC) on oral fluency development of second-language (L2) learners. Sixty-three intermediate learners of English were randomly assigned to one of three groups (two experimental groups and one control group), each consisting of 21 participants. The participants in the experimental groups completed four communicative tasks under two different instructional environments: synchronous text-based CMC environment and asynchronous text-based CMC environment. The average length of pauses, the articulation rate, the fluent-run, the phonation-time ratio and the speaking rate served as dependent variables. The study found that the participants in synchronous CMC group improved their L2 oral fluency significantly compared with their peers in the other two groups. The results also revealed the students in asynchronous CMC group obtained higher mean scores than those in the control group, although the differences between groups were not statistically significant. (Contains 2 tables and 1 figure.)