We explored whether task complexity, operationalized by the two types of writing prompts, affects EFL high school students' narrative writing in terms of syntactic complexity, lexical complexity, fluency, cohesion, and text quality. 32 intermediate EFL students who were randomly assigned to two prompt groups completed a written narrative task based on a series of sixteen pictures. Task complexity was operationalized as a bare versus frame prompt. The results indicate that the task complexity had an impact on lexical sophistication measures. The students in the framed prompt group were able to include more sophisticated vocabulary in their narratives than those in the bare prompt group. The findings are discussed in terms of the Limited Attentional Capacity Model in that the students in the bare prompt group might have prioritized meaning rather than form in order to ease attentional overload. The findings of our study could assist teachers in selecting writing prompts that have the potential to elicit the targeted features of writing performance.