Although the effectiveness of computer-mediated collaborative writing (CMCW) is confirmed by many recent studies, only a few have investigated whether linguistic knowledge and writing skills learned through collaboration can be internalized and transferred to individual writing. This study uses a pre-and post-test design to investigate the impact of CMCW in a classroom setting on Chinese EFL learners' development of writing skills and linguistic knowledge. Participants included 135 non-native English speakers with intermediate or advanced language proficiency and 45 native English speakers. They completed two online writing tasks in dyad or individually over eight weeks. The influences of task type, dyadic type and language proficiency were measured by gain scores on writing complexity, accuracy, fluency, and overall individual writing performance. Findings indicate that intermediate learners had higher gains than advanced learners, and that task type, dyadic type and language proficiency significantly support improvement in overall writing performance and fluency, but not accuracy and complexity. This study confirms the benefits of CMCW on language learning, suggests directions for future research, and offers practical applications for EFL writing teachers.