The purpose of this study is to investigate the relationship between the social capital and urban social sustainability in the urban population over 20 years old in Mazandaran province. In order to investigate the relationship between these two variables, Coleman, Putnam, Fukuyama, Arnstein, and Driskell’ s theories have been used. Data were collected using a survey method and a questionnaire tool from a sample of 400 people. Data analysis was performed using SPSS and Amos softwares. The descriptive findings of the research showed that the condition of the social capital structure and all its variables (i. e. social awareness, social trust, general norms, social participation and social cohesion) and the urban social sustainability structure (i. e. social harm, social justice, social liberty, social security and social welfare) were at a lower than average level. The explanatory findings of the research showed that social capital and all its variables had a strong direct and significant relation with urban social sustainability. Based on Pearson's correlation coefficient, social trust, social participation, social consciousness, generalized norms, and social cohesion variables had the strongest relationship, respectively. Based on the results of structural equation modeling, the theoretical model of the research based on various indices, such as CMIN / DF, RMSEA, NFI, CFI, PCFI, and etc had a reasonable fit. The results of this study confirm the empirical background and the theoretical framework of the research and determine the extent to which social capital and all its variables increase, thereby improving and enhancing the components and variables of urban social sustainability.