The present study followed a sequential explanatory mixed-methods design in two phases to investigate the effects of cognitive and metacognitive strategies on developing Iranian EFL learners’ writing assessment performance. For the quantitative stage, the researchers conducted a quasi-experimental study with a non-equivalent pretest-post-test control group design. To this aim, 41 upper-intermediate EFL university students were selected from two intact classes based on convenience sampling from the Islamic Azad University, South Tehran Branch. The classes were randomly assigned to a Cognitive-based Assessment Instruction Group (CAIG) and a Metacognitive-based Assessment Instruction Group (MAIG). A general proficiency test administered before the treatment verified the participants’ homogeneity. The Metacognitive Awareness Writing Questionnaire was administered before and after the intervention to measure the participants’ metacognitive awareness levels and self-regulation. MAIG used metacognitive strategies such as planning, monitoring, and revising to assess English essays. CAIG performed assessments based on cognitive strategies and a Writing Rubric. After the treatment, the participants took two writing posttests on rehearsed and unrehearsed topics. Results of the Mann-Whitney U test and Multivariate Analysis of Variance (MANOVA) indicated that MAIG outperformed CAIG in both posttests. Raising metacognitive awareness regarding self-regulation and metacognitive strategies could significantly enhance the participants’ writing assessment performance. In the final stage, online semi-structured interviews were performed, the findings of which were strengthened through content analysis. The study has implications for test and materials developers, EFL teachers and students, and syllabus designers in expanding their understanding of raising metacognitive awareness and its application in writing assessment.