While much research supports the benefits of computer-mediated collaborative writing (CW) in second language (L2) classrooms, the assessment of CW has received scant attention. This study proposed an assessment scheme considering both the products and processes when assessing online synchronous CW, and explored its effects on learners' interactions and perceptions of the process-and-product-based assessment approach through qualitative analyses. Through analyzing 21 dyads' online pair talk and revision histories in CW tasks, post-task reflections, and interviews, this study found that assessment approach impacted patterns of collaboration, as well as the quantity and quality of peer interaction. Also, learners reported multiple advantages of the process-and-product-based assessment approach in computer-mediated CW, including increasing fairness, promoting better performance, and facilitating effective collaboration. Further, the process-and-product-based CW assessment approach raised learners' awareness of the value of the collaboration process, motivated students to be critical collaborators, and promoted regulated learning. Nevertheless, students expressed concerns that this assessment approach might bring pressure and disrupt idea negotiation. The findings can further our understanding of the role that assessment plays in computer-mediated CW tasks and shed new light on possible ways of implementing CW assessment practices that may facilitate learning and positive learner perceptions in L2 writing classrooms.