Based on Robinson's Triadic Componential Framework and the definition of task planning by Ellis (2005), this study was carried out to find out the effects of task planning and rhetorical mode on lexical and syntactic complexity, and overall writing quality of writing production of EFL learners. Following a repeated-measures design, the present study involved 41 ELT students who learned English as a foreign language. In this study, two rhetorical modes were used and for both rhetorical modes, two writing tasks were performed. Whereas one of the tasks was carried out under strategic pre-task planning in which students had extra time and opportunity to make a plan, the other task was performed under unpressured on-line planning in which they had no time pressure or no extra special time for planning. Thus, each participant produced 4 essays, and a total of 164 essays were obtained. Each written text was separately analyzed by automated analysis tools for lexical and syntactic complexity, and also assessed through an analytic rubric for general writing achievement. The results revealed that while all the three dimensions in this study showed significant difference according to the rhetorical mode of writing, task planning had varying effects on each component.