This study investigated factors that might influence Japanese university students’ expository writing in English. We examined 70 students of low‐ to high‐intermediate English proficiency along a variety of dimensions, namely, second language (L2) proficiency, first language (L1) writing ability, writing strategies in L1 and L2, metaknowledge of L2 expository writing, past writing experiences, and instructional background. We considered these multiple factors as possible explanatory variables for L2 writing.Quantitative analysis revealed that (a) students’ L2 proficiency, L1 writing ability, and metaknowledge were all significant in explaining the L2 writing ability variance; (b) among these 3 independent variables, L2 proficiency explained the largest portion (52%) of the L2 writing ability variance, L1 writing ability the second largest (18%), and metaknowledge the smallest (11%); and (c) there were significant correlations among these independent variables. Qualitative analysis indicated that good writers were significantly different from weak writers in that good writers (a) paid more attention to overall organization while writing in L1 and L2; (b) wrote more fluently in L1 and L2; (c) exhibited greater confidence in L2 writing for academic purposes; and (d) had regularly written more than one English paragraph while in high school. There was no significant difference between good and weak writers for other writing strategies and experiences. On the basis of these results, we propose an explanatory model for EFL writing