This study evaluated the effectiveness of an instructional programme designed to improve the regulatory fit of, and lessen the working memory demand upon, students in the process of second language (L2) writing. Drawing on the theoretical understanding of regulatory focus theory (RFT) and cognitive process of writing, a quasi-experiment with control group was designed and testified on 138 secondary school students in Hong Kong over a period of 16 weeks. The findings suggest that the aforementioned instructional design is effective in promoting students’ writing performance in terms of quality, creativity and accuracy. The pretest-posttest gains exhibited in the experimental group suggest that the students were able to internalize the writing instruction, self-regulate their writing processes, and benefit from it. In hierarchical regression, working memory explained unique variances in the control group but not in the experimental group, perhaps because the writing instruction rendered working memory constraint unimportant. The results not only offer empirical support for the application of RFT in education settings, but also inspire teachers with ways to improve students’ writing performance by fine adjustment in writing activities and grading system. / published_or_final_version / Educational Psychology / Master / Master of Social Sciences