This study investigated the effects of planning on second-language written production with regard to proficiency level, and task type. The participants were 157 Korean learners of English as a foreign language attending a four-year university in Korea. They were asked to complete two different types of writing tasks (Expository writing task and Argumentative writing task) in different planned conditions (Individual Planned Condition and Collaborative Planned Condition) over a two-week period. In the Individual Planned Condition, learners were given 10 minutes for individual planning in the prestructured task sheet and then asked to write an essay for 30 minutes. In the Collaborative Planned Condition, learners were allowed to interact with a peer during planning and they required to independently complete an essay. Participants' written products were evaluated on five analytic measures covering the areas of Content, Organization, Language in Use, Grammar, and Mechanics. The results of MANOVA tests indicated that the planned condition had an impact on learners' written performance in both tasks. Individually considered, learners in the Collaborative Planned Condition were able to achieve significantly higher scores in all the analytic features in Task 1 (Expository writing task). In contrast, there were no significant mean differences between two conditions in Task 2 (Argumentative writing task). The results also indicated that proficiency had influenced learners' written performance in both tasks. The proficiency effect was consistently found throughout the analytic scores Task 1 and Task 2. However, the interaction between condition and proficiency was not found in the two tasks. The results of repeated measures for the effect of task type revealed that significant mean differences were only found in the Mechanics section. It is concluded that Korean EFL learners' written performance was affected by planned condition and proficiency, but to only a small degree by the nature of task type with regard to the five analytic features. The findings of this study help broaden the understanding of second language learners' cognitive writing process involving planning. In addition, the results have pedagogical implications as well as theoretical implications in second language writing and relevance to second language writing assessment.