Female sex workers (FSW) are high-risk for HIV infection. The authors conducted a quasi-experimental study by providing peer education on 110 FSWs, and routine HIV education on 120 controls, in Surakarta Indonesia. An additional qualitative study was performed to explain the contextual factors contributing to the effectiveness of peer education. The difference in difference approach showed that peer education improved knowledge, acceptance, and utilization of female condom. The improvements were related to the clients' perceptions, access, and available interactive communication with the peer educator. The peer education program should be enhanced to support the prevention of HIV/AIDS.