This quasi-experimental study investigates the effects of pre-writing (Pre-EI) and post-writing explicit instruction (Post-EI) on L2 learners' overall writing accuracy and errors at different severity levels. Situated within process-genre-oriented writing classrooms, a total of three intact groups (N = 101) were designated as two experimental conditions (Pre-EI and Post-EI) and a control group (no explicit instruction). Overall, the improvement by the two treatment groups compared to the control group suggested that both Pre-EI and Post-EI were effective in improving learners' overall writing accuracy and reducing level 2 and level 3 errors. However, the results of between-group comparisons revealed the superiority of Post-EI. These findings indicated that writing accuracy was conditional to both timing and availability and provided a more nuanced understanding of the differential effects of Pre-EI and Post-EI on writing accuracy. Implications for L2 writing pedagogy and future studies are discussed.