It is argued that tasks with different structures yield different performances in terms of accuracy, fluency and complexity. The present study is thus an attempt to investigate the impact of task structure on second language task performance. Thirty two upper-intermediate Iranian learners of English performed two narrative tasks (Structured vs. Unstructured) based on cartoon scripts. The performances, then, were recorded, transcribed and coded to measure the accuracy, fluency and complexity of the participants' performances. Next, the paired samples t-test was employed to analyze the collected data. The results of the statistical analysis revealed that task structure had no effect on the accuracy, fluency and complexity of performance. Findings have pedagogical implications in the area of syllabus design.