OBJECTIVE: To study the prevalence of human immunodeficiency virus (HIV), sexually transmitted infection (STI), risk behavior and the sexual networks among men who have sex with men (MSM) in Taizhou city, Zhejiang province. METHODS: A cross-sectional study was applied with venue-based sampling in 2 MSM gathering sites in Taizhou. 'Informed Consent' principle was applied and MSM were studied through a structured questionnaire. Blood samples were collected from those who accepted free and confidential HIV/STI counseling and then tested for HIV, syphilis, HCV and HSV-2 antibodies with ELISA. HIV positive sera were certified with western blot. RESULTS: 106 MSM were investigated and 97 qualified questionnaires were collected. 25.0% (23/92) of these MSM have ever had 1 female sex partner while 47.8% (44/92) had 2 or more. 14.3% (13/91) of them reported having had 1 male partner who had engaged in anal sex and 80.2% (73/91) had 2 or more. 22.1% (19/86) of them had participated in group sex but 62.5% (55/88) of them did not always use condom when having anal intercourse. 15.1% (14/93) of them had 1 oral sex partner while 75.3% (70/93) having 2 or more. 38.9% (37/95) of them had sex with female sex worker, and 35.5% (33/93) had sex with male-to-male sex worker. 15.3% (13/85) of them had once been male-to-male sex worker themselves. 3.9% (3/77) of them were found HIV positive in blood tests, with 24.7% (18/73) positive of syphilis, 15.1% (11/73) positive of HSV-2 but HCV appeared to be negative. 46 cases reported their egocentric recognition networks, with mean degree of 5.91 (ranging 0 - 10), and mean density of 0.548 (ranging 0.000 - 1.000). 43 sexual networks were identified, with mean degree of 2.70 (ranging 0 - 10), and mean density of 0.246 (ranging 0.000 - 1.000). CONCLUSION: Risk behaviors, such as multiple sex partners, low proportion of condom use and commercial sex engagement, both with heterosexuals and homosexuals, were extensively existed among MSM in Taizhou, and the prevalence of HIV/STI was relatively high. Their sexual networks seemed complicated but there might be in place of some substantially isolated MSM groups with high risk of HIV/STI infection. More study should be applied to identify the relationship between sexual networks and HIV/STI transmission.