This paper investigates how task complexity and cooperative learning influence Korean EFL middle school students’ narrative writing in terms of fluency, accuracy, and complexity, and their writing ability development when performing a three-stage task with differentiated complexity by reasoning demands of picture sequence. In the task, students were given and read a model text individually or in group to find out problems of their own writing and solutions with note-taking between writing a draft and rewriting it, assuming model texts would play a role of a feedback provider and cooperative work would lessen cognitive demands of complex tasks. The analysis on fluency, accuracy and complexity of 68 students’ essays reveals no significant group differences and interactional effects by task complexity and cooperative learning, except for mean length of T-unit by task complexity and numbers of T-units per minute by interactional effects of task complexity and cooperative learning; yet, those who performed a more complex task showed lower fluency, accuracy, and complexity and the cooperative learning group outperformed the individual learning group in mean length of error-free T-unit, syntactic accuracy and lexical diversity. The statistical analysis of the writing score differences between the pre- and post-test displays no significant group variations and interactional effects, though the group performing a more complex task and individual learning group outperformed their counterparts. The survey of activity preference illustrates cooperative learning was noticeably preferred by those who performed a more complex task, which implies group work would lessen cognitive demands of complex tasks.