The present study examines the effects of manipulating cognitive task complexity on high school English learners’ narrative and persuasive writing. Participants were 156 high school students. They were divided into four groups. Each group was given one of four different types of writing that were classified based on their genres (narrative vs. persuasive writing) and dimensions of task complexity (resource-directing vs. resource-dispersing). All participants completed both simple and complex writing tasks for their assigned type of writing. Participants’ written products were measured in terms of complexity, accuracy, and fluency. The results revealed that increased task complexity, along with the resource-directing dimension, somewhat positively affected the complexity, accuracy, and fluency of both narrative and persuasive writing. However, increased task complexity, along with the resource-dispersing dimension, showed differential effects of cognitive complexity on participants’ written products between the genres. It resulted in decreased scores in fluency and accuracy, and had no significant impact on the complexity of narrative writing. As for persuasive writing, on the other hand, it lowered the fluency, increased the complexity, and had no impact on the accuracy. The pedagogical suggestions drawn from the results are provided along with the limitations of the study.