Background and purpose: The first conflict with disability takes place in the family, which is associated with certain socio-economic and psychological aspects. Social capital is one of the social factors that affects health. The aim of this study was to identify the relationship between social capital and mental health of families with disabled members in Tehran, Iran. Materials and methods: This cross-sectional study is a secondary analysis conducted based on the data obtained from the Urban Health Equity Assessment and Responses National Tool Project, which was administered in 2011. The study population included 3571 families with disabled members, who completed the mental health questionnaire. Data analysis was conducted using structural equation modeling in SPSS and AMOS. Results: The participants included 48.4% males. Among the participants, 66.8% were not in good state of mental health. The values for bonding and total social capital were lower than the standard levels. The results of the structural equation modeling showed a significant relationship between social capital and mental health of the families with disabled members. Conclusion: Social capital is a significant and effective factor in the mental health of families with disabled members. Therefore, appropriate conditions are needed to improve social participation and public trust and increase emotional, mental, and social support to promote the state of mental health in involved families