This study is to describe the nature of planning as a cognitive writing process in L2 writing. It aims to examine how Korean EFL learners generate ideas and organize them. It also investigates the relationship between planning and the final product. Given a worksheet for planning, 39 university students were asked to write an argumentative essay during class. Based on their planning notes, five brainstorming types were identified: using the prompt/writing the position, mini-outlining, listing, mind mapping and free writing (in the order of frequency). In addition, the dominant use of L1 was found both in the brainstorming and the outlining. It was found that there was no statistically significant correlation between the amount of brainstorming and the quantity and quality of L2 writing. Only the amount of brainstorming in L2 had a statically significant correlation with the quantity of L2 writing (but not with the quality of L2 writing). In the case of outlining, a statistically significant correlation was found between its amount and the quantity of L2 writing. However, no statistically significant correlation was found between the amount of outlining and the L2 writing quality.