Various studies suggest that it is important to determine students' personality charecteristics and assertiveness and make them aware of these differences in order to improve learning process and outcomes. This study aimed to find out how much of the variance in postgraduate students’ assertiveness is explained by five personality characteristics (i.e., Openness, Neuroticism, Extroversion, Agreeableness, and Conscientiousness) and which one of these characteristics significantly predicts the students’ assertiveness. To this end, 66 postgraduate students within the age range of 22 to 35 participated in this descriptive correlational study. They were majoring in English language teaching at Islamic Azad University,Tabriz Branch, Tabriz, Iran. The College Self-expression Scale (CSES) and Big-Five Personality Inventory were used to assess the students’ level of assertiveness and their personality characteristics, respectively. In order to answer the research questions, a multiple linear regression analysis was carried out. The results revealed that about 14 percent of the postgraduate students’ assertiveness is explained by five personality characteristics and out of them, only Extroversion has a significant contribution to their assertiveness. The pedagogical implications are presented for teachers, students, syllabus designers, and materials writers.
