The instruction of metadiscourse markers to L2 writers has been recommended by some scholars to assist them in employing a certain tone in persuading readers. Nonetheless, there is a dearth of research investigating the effect of process-based instruction on hedging and boosting devices to L2 learners. To fill this gap, the present study aimed at investigating the effect of explicit instruction of hedging and boosting devices through a process-based approach on the achievement of L2 learners in academic writing. By comparing the results obtained through teaching 115 Iranian IELTS candidates who benefitted from: (1) a process-based writing instruction with its focus on hedging and boosting; (2) mere process-based writing instruction; and (3) conventional writing instruction, the researchers found that the process-based approach to teaching writing with its focus on hedging and boosting resulted in a significant effect on L2 learners' academic writing achievement. The findings of this study may have implications for EAP teachers, syllabus designers, and decision makers in designing EAP writing courses.