In the last few decades, tasks as the basic unit in syllabus designing process, and the cognitive complexity of tasks as a fundamental criterion in task sequencing, have attracted the attention of language researchers. At the same time, collaborative task performance has been encouraged due to the researchers' focus on the sociocultural theory proposed by Vygotsky (1978). The present study aimed to investigate the effect of task cognitive complexity on Iranian EFL learners' individual and collaborative written task performance regarding complexity, accuracy, and fluency. To fulfill such a purpose, 70 male and female Iranian intermediate EFL learners were selected as the participants through Oxford Placement Test, from among the undergraduate students of Islamic Azad University of Arak, who had passed two general English grammar courses in two succeeding semesters and were participating in Advanced Writing Course at the time of conducting this research. They were asked to perform some writing tasks both individually and collaboratively, while the cognitive complexity of the tasks (i.e. the reasoning demands and prior planning) was evaluated by the researcher. The analysis of their written productions revealed that when writing individually, with no planning time available, Iranian intermediate EFL learners created more complex texts in their complex tasks and more fluent texts in their simple tasks, and when writing collaboratively, with some planning time available, they produced more fluent texts in their complex tasks. تفاصيل المقالة