This study aimed to investigate the effects of self-assessment implementation on ninth grade students' writing competence and motivation. This study was designed as a quasi-experimental study and obtained quantitative data from questionnaires and writing scores. Qualitative data was gathered from semi-structured interviews conducted with the students. They completed an English writing motivation questionnaire and a pre-test writing task, and then they graded their pre-writing task by utilizing the rubric explained in detail. During the study process, the students completed 14 writing tasks, including pre- and post-test writing tasks, and assessed their own writing works with the simplified rubric. Meanwhile, this study also graded the students' writing tasks with the same rubric as the students. Before the completion of this study, the students took the same motivation questionnaire and writing task that they completed at the beginning of the study. The findings revealed that self-assessment increased the participants' self-awareness and writing scores. On the other hand, the self-assessment implementation did not lead to a significant difference in the students' English writing motivation, although the participants stated during the interviews that they found self-assessment useful and effective to improve their writing skills.