Objective: This study aims at investigating social trust and its determinants in Sabzevar city. The dependent variable social trust has been measured through two dimensions: thick trust and thin trust. However our emphasis is on the second one. Hypotheses have been extracted from two theories: social capital theory of Putnam and institutional theory of Rothstein and Stolle.Method: Through survey method and questionnaire technique, a sample of 600 individuals at the age of 18-65 was interviewed.Findings: Results indicates that there is a significant relationship between independent variables including discrimination experience, corruption, feeling social security, institutional trust(these four variables are based on institutional theory) as well as age, and thin trust. These relationships are negative for the first two variables and positive for the rest. However there is not a significant relationship between variables such as participation in formal groups, using media, participation in informal groups, education, income, and job rank. In multivariate analysis using stepwise regression method, four variables entered in the model orderly: institutional trust, corruption, social security and age. The four variables explain 53 percent of the variance of dependent variable- thin trust.Result: All together we found out the Rothstein and Stolle's institutional theory is more suitable to explain social trust than Putnam's social capital theory.