Background: One mode of HIV transmission is through the high-risk behaviors among people who are HIV-positive. It is, therefore, exceedingly important to understand the factors predicting preventive healthy behaviors regarding HIV transmission in this population. Objectives: The aim of this study was to evaluate the efficacy of the extended Health Belief Model (HBM) in predicting preventive behavior changes among HIV-infected patients in Iran. Patients and Methods: In a cross-sectional study, the census sampling was applied and 139 HIV-positive patients in Saravan, Zarandieh, and Sarbaz cities of Iran were selected to participate in this study. In total, 131 participants completed and returned the anonymous self-administered questionnaires. The questionnaire included demographic questions, HBM constructs, and the questions of Zimet social support. Data analysis was performed using SPSS18. P-values less than 0.05 were considered significant. Results: The HBM constructs with and without social support accounted for 62% and 56% of the behavior variance, respectively. In the first model (HBM construct alone), perceived barriers, self-efficacy, and benefits were the significant predictors. In the second model (HBM developed with social support), perceived barriers, social support, self-efficacy, and benefits were the better predictors of preventive behaviors, in sequence (P < 0.05). Conclusions: The findings indicated that the extended model of HBM predicted preventive behaviors better than the original HBM did. However, more studies are needed to examine the efficacy of the extended HBM in predicting preventive behaviors.