The present study investigated the potential of writing in English as a foreign language (EFL) for language learning by manipulating cognitive task complexity based on related models and hypotheses. English essays written by 59 Chinese postgraduate EFL students from different subject areas were analysed with reference to writing complexity, accuracy, and fluency (CAF). Results showed that task complexity had no significant effect on EFL learners' lexical complexity but had an influence on their syntactic variation in EFL writing. Findings suggest that manipulating writing task complexity could be a feasible means to promoting and enhancing EFL learners' language learning. Such findings might broaden our understanding of the relationship between EFL writing and language learning in an EFL learning context. The interplay of EFL writing and EFL learning is also pedagogically relevant to those who are interested in appropriately sequencing tasks for more effective language teaching.