We examined the mediation effects of a family-based substance use prevention program on a sample of Asian American families. These families were randomized into an intervention arm or a nonintervention control arm. Using path models, we assessed the effect of the intervention on adolescent girls’ substance use outcomes at 2-year follow-up through family relationships and adolescent self-efficacy pathways. Bias-corrected bootstrapping strategy was employed to assess the significance of the mediation effect by evaluating the 95% confidence interval of the standardized coefficient. The results showed that receiving the intervention exerted a positive effect on girls’ family relationships at 1-year follow-up. Such an improvement was associated with girls’ increased self-efficacy, which in turn led to girls’ decreased alcohol use, marijuana use, and future intention to use substances at 2-year follow-up. Considering the diverse cultural backgrounds, as well as languages, nationalities, and acculturation levels under the umbrella term Asian Americans, we demonstrate that a universal Web-based intervention that tackles the theoretical- and empirical-based risk and protective factors can be effective for Asian Americans. Despite its generic nature, our program may provide relevant tools for Asian American parents in assisting their adolescent children to navigate through the developmental stage and, ultimately, resist substance use.