Concerning the limited capacity of working memory and drawing on Skehan’s (1198) Limited Attentional Capacity Model, this paper explored the effect of planning time on Iranian EFL writers’ performance in the different contexts of elicitation. Forty-one Iranian EFL learners in two contexts (university and institute) were asked to write in an argumentative genre under different planning time conditions (pre-task planning and online planning). The results from evaluating the participants' textual outputs in terms of three language production aspects, namely complexity, accuracy, and fluency, revealed a significant effect of planning time on the learners' written performance. The pre-task planning, by reducing the cognitive loads on working memory, helped the learners to have better performance in terms of fluency during the task completion. However, the planning process did not exert statistically significant differences on the complexity and accuracy of the written productions. Given the context of elicitation, the results indicated a significant effect on the participants' written performance in terms of complexity and accuracy. Moreover, significant interaction effects were found between planning time and context. The findings of this article shed more light on understanding the effect of the planning process as a major cognitive component of the writing process in second/foreign language learning. Furthermore, the results of the study have pedagogical implications for utilizing planning process to improve writing performance of the language learners.