This study investigates the direct and/or indirect effects of some cognitive (working memory capacity) and affective (writing anxiety and writing self-efficacy) variables on the complexity, accuracy, and fluency (CAF) of second language (L2) learners' writings. To achieve this goal, 232 upper-intermediate English learners performed an automated version of a working memory capacity task (A-OSPAN) and a timed narrative writing task in L2. Furthermore, participants were asked to complete two self-report questionnaires. The proposed path model adequately fitted the data, and results of path analyses indicated the following: All three measures of L2 writing were directly predicted by learners' writing self-efficacy; writing self-efficacy affected CAF indirectly through writing anxiety; the direct paths from writing anxiety to all measures of L2 writing were negatively significant; higher working memory spans directly predicted higher L2 writing scores regarding complexity and fluency, but negatively affected learners' accuracy scores. Based on these findings, the author discusses techniques for enhancing learners' writing self-efficacy, reducing their anxiety, and helping them make efficient use of working memory resources.