This study investigated cognitive factors that might influence Chinese EFL learners’ argumentative essay writing in English. The factors that were explored included English (L2) language proficiency, Chinese (L1) writing ability, genre knowledge, use of writing strategies, and working memory capacity in L1 and L2. Data were collected from 136 university students who received a battery of tests in two sessions. The tests consisted of timed essay writing tasks in L1 and L2, post-writing questionnaires for genre knowledge and use of strategies in the writing process, a timed grammaticality judgment task for L2 grammar knowledge, a receptive vocabulary test and a controlled-production vocabulary test for L2 vocabulary knowledge, and working memory span tasks in L1 and L2. Quantitative analyses using correlations, paired-samples t-test, analysis of variance and multiple regression revealed that L2 language proficiency is the most important predictor of L2 writing, followed by genre knowledge and L2 writing strategies. L1 writing ability and working memory capacity have slight impact as explanatory variables for L2 writing performance in the timed essay writing task.