One of the most significant current discussions in writing is self-assessment and peer assessment. This study aimed to investigate the impact of writing self-assessment and peer assessment on autonomy and metacognitive awareness of Iranian EFL learners. One hundred and twenty participants were selected using convenience sampling. Four instruments were used in this test: (1) Nelson placement test; (2) Metacognitive Awareness Inventory (MAI); (3) Zhang and Li’s autonomy questionnaire; and (4) rubric. A quasiexperimental design was used in this study. They were divided into two experimental groups, self-assessment (N = 40) and peer assessment (N = 40), and one control group (N = 40). Collected data were tested and compared using covariance analysis (ANCOVA). Results from the tests indicated that both self-assessment and peer assessment are effective ways to improve autonomy and metacognition awareness of EFL learners in the completion of writing tasks. Findings also showed that self-assessment was more effective than peer assessment and the results showed that the types of assessments increased the learners’ knowledge in the writing, teaching, lexicography, spelling, grammar, and similar models and were significantly effective in developing their writing skills. The findings of the present research study might have some implications for researchers, instructors, language teachers, and language learners.