Though many studies have been conducted on the effects of self-assessment on L2 learning over the last decade, none has investigated how self-assessment reports lead to improving L2 learning. Thus, this study aims to disclose how self-assessment reports improve Iranian high school students' writing skills and their perceptions of the effectiveness of self-assessment reports in cultivating writing skills. For this purpose, the researchers selected o ne intact grade 11 class (n = 21) at Shahed public high school in Borujerd, Iran. During the instruction lasting 15 sessions held twice a week, a self-assessment report was designed based on Nunan's (2004) template and given to the students to assess their writings weekly. Then, a focus group interview was run with six students. Findings documented a significant improvement in the students' writing skills in terms of content, language, organization, and task requirements. Additionally, the focus group interview results yielded four themes: raising students' awareness of evaluation criteria, making students more self-regulated, letting students have a voice in their academic destiny, and increasing students' motivation to write. In light of the findings, a number of conclusions are drawn, and several implications are put forward.
