Female sex workers (FSWs) have been identified as one of the high-risk population groups for HIV and STI infections; they play a potential bridging role in the spread of the STI/HIV epidemic from specific high-risk populations to the general population. A cross-sectional study using mixed methods (quantitative research combined with qualitative research) was conducted to determine the prevalence of STI/HIV infections and assess the levels of STI/HIV-related knowledge, and attitudes towards STIs/HIV and sexual behaviours among FSWs in Ba Ria – Vung Tau, Vietnam. Quantitative research methods were used for an epidemiological survey to recruit 420 FSWs in two cities and six districts of Ba Ria – Vung Tau province, Vietnam. The survey explored the STI/HIV prevalence, socio-demographic characteristics and risk factors affecting FSWs. The recruited participants were interviewed face-to-face, using a structured questionnaire, and then tested for HIV, Syphilis, Gonorrhoea, and Chlamydia. Qualitative research methods using in-depth interviews with a subsample of 45 FSWs supplemented the quantitative research findings, especially in relation to different perspectives of risk behaviours among FSWs.