Knowledge sharing is one of the main components of knowledge management that has a prominent role in universities for generating new knowledge and competitive advantage. Social trust has the mechanism of facilitating in knowledge sharing. Social trust is a mediator mechanism in sharing knowledge. Thus, the main purpose of this research is a social distrust in knowledge sharing as a social problem. The method of this study was descriptive-survey. The population of this study was graduate students of Tabriz University and with using of Cochran formula and proportional Sampling 650 person selected as a sample size. Reliability coefficients for social trust (0. 776) and dependent variable of research is (0. 812) This suggests that the selected items are suitable for measuring the variables of the research. The data gathering tool was a social trust and knowledge sharing questionnaire that was analyzed using SPSS software version22. Findings show a positive and significant relationship between social trust and student's knowledge sharing(r=0/556). There is also a positive and significant correlation between social trust components with knowledge sharing. The results of the correlation test indicate that the age and the score of students have a positive and significant relationship with knowledge sharing. T-test analysis show that knowledge sharing does not have significant difference based on gender of students. The results of the regression show that the component of social trust explain39% of knowledge sharing variance. Meanwhile, the role of toward collaboration is more than other factors. Therefore, in order to increase the level of knowledge sharing among graduate students the factors affecting social trust should be seriously considered.