Abstarct: The decrease in social capital in urban areas can lead to a decrease in the residents' quality of life. The characteristics of the physical environment of neighborhoods have an important effect on residents' social capital. Despite numerous research about social capital, the impact of the physical environment of neighborhoods on social capital has been less investigated. The aim of this study is to determine the effects of the physical environment on residents' social in neighborhoods. Firstly, using the Content Analysis, conceptual and operational definitions of the research variables were extracted and conceptual frxamework was obtained. The statistical population of the research were residents of two neighborhoods in Mashhad. The sample size was determined using statistical power with GPower software. Perceived characteristics of the physical environment and social capital were collected and a researcher made questionnaire was used. Random sampling was utilized to select 532 adult residents. The reliability of the questionnaire was confirmed using factor analysis and Cronbach's alpha and omega coefficients. Experts confirmed the content validity of the questionnaire. The construct validity of the questionnaire was assessed and confirmed using factor analysis. Complete structural equation modeling was used to test the research hypotheses. Data was analyzied using SPSS 26 and AMOS 24 software. Five dimensions for social capital including trust, ethnic and religious cohesion, sense of attachment, attitudes and participation. Among the four factors of the physical environment- " physical activity and recreation enabling environment ", "security", "accessibility and mixed uses", and "aesthetics"- two factors of accessibility and mixed use, and aesthetics had a positive and significant effect on social capital and explained 19% of the changes in social capital. The effect of neighborhood aesthetics on social capital was stronger than accessibility and mixed uses. The difference of the social capital between the old (Balakhiyaban) and the new (Hijab) neighborhoods was not significant. Among the demographic factors, the number of children and the length of residence in neighborhoods had a positive and significant impact on social capital. Also, age performed a moderating role in the relationship between physical environment and social capital, and the effect of physical environment on social capital was stronger for young rather than middle-aged people. According to the results, strategies were suggested to increase social capital in neighborhoods.