This study explored the impact of task complexity on task performance of Iranian lower-intermediate and advanced language learners. It also investigated how working memory was related to task performance and mediated the influence of complexity conditions on language performance. Task complexity was operationalized by designing differing tasks along the +/- reasoning and the +/- few elements dimensions. Eighty Iranian EFL learners (40 lower-intermediate and 40 advanced) carried out argumentative tasks which differed in complexity level. Working memory capacity was measured by applying the Persian translation of Wechsler's (1987) working memory test, and task performance was measured in terms of accuracy and fluency. The results revealed that for lower-intermediate learners, task complexity led to decrease in accuracy in the complex tasks, while fluency was boosted in simple task condition. For advanced learners, task complexity resulted in improved accuracy, while fluency decreased in complex condition. The results of multivariate analyses revealed that learners' language performance in the complex group significantly differed from that of the simple group on the combined dependent variables for both lower-intermediate and advanced learners. There was no significant correlation between working memory and any performance measures.