This study determines collaborative writing factors affecting English as a foreign language (EFL) students' writing performance (WP). The model draws on Bandura's social cognitive learning theory, focusing on environmental factors, individual beliefs, and behavioral factors. The questionnaires were delivered to 85 EFL students at a university in Vietnam. The statistics show that the standardized root mean squared residual index is 0.060, meaning a good model fit. In particular, the path regressions indicate that WP is affected directly by behavioral factors (63.7%) and indirectly by environmental factors (61.4%) and individual beliefs (24.1%). Regarding the collaborative writing environment, WP is indirectly affected by interaction with peers (37.3%), interaction with teachers (16.6%), and attitude toward the course (12.3%); however, the indirect effect of the use of technology on WP is not statistically significant. Finally, the author discusses some major administrative measures to stimulate causative factors, which will improve EFL students' WP.