Despite the solid evidence that online collaborative writing (CW) tasks benefit language learning, little is known about whether the computer-mediated communication (CMC) modality may impact the potential of online CW tasks in foreign language classrooms. This study examined the effect of task modality on the utility of CW tasks in language classrooms. Fourteen Chinese as a foreign language (CFL) learners completed Google Docs-based CW tasks in two modalities: A synchronous CMC modality entailing video-chatting in Zoom and an asynchronous CMC modality where learners interacted in Google Docs with delayed time. Through analyzing multiple data sources (i.e., peer interaction, written products, editing logs, interviews), we found that, while CMC modality did not impact writing performance, CFL learners had more language learning opportunities and more positive learning experiences in the synchronous mode than in the asynchronous one. The findings revealed multiple challenges that require attention when implementing asynchronous CW tasks in teaching.