The purpose of this study is to investigate the effects of no planning and online planning on complexity, accuracy, and fluency (CAF) in L2 spoken and written tasks. The study was designed into 2x2 with two independent variables: planning conditions (no planning and online planning) and task modality (speaking and writing). First, 80 Korean EFL undergraduates performed two tasks in a laboratory setting: a proficiency-assigning narrative spoken task and a randomly assigned main task. The main task had four different conditions: no planning in speaking, online planning in speaking, no planning in writing, and online planning in writing. The participants' spoken and written performances were measured and analyzed for syntactic complexity, accuracy, and fluency. The findings demonstrated that the no planning (NP) group improved accuracy and fluency significantly more than the online planning (OP) group in the spoken task and that the NP group improved their fluency more than the OP group in the written task. Additionally, in both planning conditions, the written task increased accuracy more than the spoken task, whereas the spoken task increased fluency more than the written task. It is hoped that these findings will facilitate understanding of the supportive role of planning in task manipulation in terms of L2 learners' limited attentional resources and its relationship with linguistic performance.