It is common for individuals with diverse demographic backgrounds to collaborate through computer-mediated communication (CMC) technologies. Groups with internal diversity are typically considered to be advantageous to group performance due to the presence of different perspectives and the potential to stimulate new ideas. However, intergroup conflicts can also occur in diverse groups, especially for groups with imbalanced composition. Previous studies have pointed out that minority members often suffer from unequal participation and performance pressure, which may further decrease group outcome. Since CMC tools facilitate online collaboration, it is necessary to understand how group composition interacts with the affordances of CMC tools on group collaboration. In this study, we tested three gender compositions (female-majority, equal-gender-composition, male-majority) with two communication contexts (video-text, text-only) and found that both gender composition and communication medium influenced group collaboration. Design implications for online collaboration are provided based on our findings.