Feedback plays a fundamental role in writing development. The present study seeks to investigate the impact of teacher's written feedback on the writing performance of Iranian undergraduates. The subjects were 400 students majoring in the fields of English language translation and English language literature in four universities, namely Shahaid Bahonar, Vali-e-Asr, Isfahan and Shiraz, in Iran. Writing tasks, questionnaires and interviews were used in the process of the present study. Mixed methods research design was adopted in this study. Therefore, this research used the triangulation of data collection techniques. For data analysis, the thematic coding was used for analyzing semi-structured interview. In the writing task, inter-rater agreement (Cohen kappa) was used and the Pearson correlation coefficient was run for analysis of the questionnaire. The result shows a significant relationship between teacher's written feedback and students' writing performance.