Robinson's (2001, Task complexity, task difficulty, and task production: Exploring interactions in a componential framework. Applied Linguistics, 22(1), 27-57. doi: 10.1093/applin/22.1.27) Cognition Hypothesis states that requiring second language (L2) learners to engage in tasks of increasing complexity may promote greater quality of language production. However, few studies have explored the impact of increasing task complexity on learners' development of L2 oral performance over time. The present study employed a pre-test/post-test design to investigate how engaging in tasks with different degrees of complexity impact L2 oral performance development with sixty Korean learners of English. Task complexity was manipulated based on the [+/-reasoning demands] variable following Robinson's Triadic Componential Framework (2001). To measure the effects of task complexity on L2 oral performance, learners were assigned to one of four conditions: a) simple task, b) +complex task, c) ++complex task, and d) no task. The pre- and post-test results were analysed for complexity, accuracy, and fluency. Results indicated that L2 oral performance under complex task conditions led to greater syntactic complexity and fluency over time.