This study investigates the effects of the cognitive complexity of tasks and pair proficiency on task production in Synchronous Computer Mediated Communication. In particular, this study explores whether task complexity and EFL learners` proficiency have an effect on the accuracy, the fluency, and the use of strategy of task production in SCMC. 90 Korean high school EFL learners collaboratively completed the cognitively simple and complex tasks, using a browser-based chat program. Based on learners` English proficiency, each dyad was composed of three different sets: High-High pair, High-Low pair, and Low-Low pair. The data were analyzed by a 2 X 4 analysis of variance (ANOVA). Error-free C-units, words per C-unit were served as dependent variables. Results show that the cognitive complexity of a direction-giving map task significantly affects task production. Pair composition also significantly affects the number of error-free C-units, words per C-unit and the use of strategy. This study confirms a previously reported increase in quantity of strategy used by students of the pairs with different English proficiency levels, compared to other pairs with similar English proficiency levels. The findings offer pedagogical implications for designing task-based synchronous interaction to facilitate Korean EFL learners` accuracy, fluency and the use of strategy.