The purpose of this study was to investigate whether the core members' social capital was associated with individually perceived and externally evaluated prestige and cooperation among the HIV/AIDS-related civil society organizations (CSOs). To accomplish this, a cross-sectional study using multistage sampling was carried out in eight provinces of China. Data were collected from the 327 core members via questionnaires and self-evaluated performance of the respondents were evaluated and measured. The interviews were conducted with all core members and the supervisory staff of the local Centers for Disease Control and Prevention. Multivariate logistic regression analysis indicated that social support (adjusted odds ratio [aOR] = 1.87) and organizational commitment (aOR = 1.57) were significantly associated with a higher odds of prestige performance in self-evaluation. Furthermore, social support (aOR = 1.65), trust (aOR = 1.33), and organizational commitment (aOR = 1.52) were significantly correlated with cooperation performance. Trust was positively associated with the cooperation performance on external evaluation. These findings may provide a new perspective on challenges that the CSOs face in response to a growing HIV/AIDS epidemic in China. Social capital may increase performance and accelerate organizational growth, ultimately improving HIV/AIDS prevention and care.