In this correlational study, 54 MA students were selected out of 68 initial participants majoring in TEFL at Khatam ol-Anbia University, Tehran, Iran. The homogeneity of participants was calculated through administration The Cambridge Proficiency Test. Then, participants took a motivation test. After that, a topic was given to them to write an essay. All important parts of essay including thesis statement, body paragraphs, conclusion, outlining, and coherence was already taught to the participants. In addition, they were asked to read their own writing and score based on Writing Scoring Rubric modified. In order to avoid subjective rating and ensure the intra- rater reliability of the assessment, these writing samples were scored twice based on the same scale that the students utilized in self-assessment. The results revealed that there was not statistically significant relationship between motivation and students` self-assessment. Moreover, the relationship between self-assessment and teacher’s rating of students` writing ability was statistically significant. The findings have some implications for language teaching and assessment.