Abstract：Abstract: Task complexity is an important consideration when implementing the Focus on Form principle of task-based language teaching. In this study, an interpretive sequential design was adopted to examine the effects of task complexity and working memory capacity on L2 writing performance, as well as the role of learner agency. The quantitative investigation showed that task complexity and working memory capacity had significant main effects on the linguistic accuracy and complexity of the subjects’ linguistic performance, but there were non-significant interaction effects between them. The qualitative survey revealed that driven by extrinsic motivations, many subjects with low working memory capacity took a variety of strategies to adapt the complex task, rendering the performance differences in terms of the two levels of task complexity and two levels of working memory non-significant. It can be seen that task design needs to take into account both cognitive and social factors involved, so as to maximize the Focus on Form effect that tasks are assumed to exert. Key words: task complexity; working memory capacity; linguistic performance; agency
