Objective: To investigate the behaviors at high risk of human immunodeficiency virus(HIV) infections and related factors among men who have sex with men(MSM) in Yangzhou,Jiangsu province. Methods:A cross-sectional study with snowball sampling method was conducted from May 2008. The researchers carried out face to face questionnaire about the knowledge attitude and behavior on acquired immune deficiency syndrome (AIDS) among MSM. Blood samples were collected for test of HIV and other sexually transmitted diseases (STD) infection. Results:Among 300 respondents,82.7% of them had homosexual acts in the past six months and 79.4% had multiple homosexual partners;48.7% had heterosexual sex in the past six months and 21.2% had multiple heterosexual partners. The proportion of continuely condom using in homosexual sex in the past six months was 52.0%,while the proportion in heterosexual sex was 21.2%. Laboratory test results showed that the prevalence of HIV infection was 7.3% (22/300) and the prevalence of syphilis was 22.0% (66/300),and no HCV-positive sample were found. In multivariate logistic regression model,looking for sex partners through the internet(OR = 1.93,95% CI:1.01-3.67) and having homosexual sex with regular partners(OR = 0.45,95% CI:0.25-0.79) were significantly associated with the continuely condom using in homosexual sex in the past six months;the infection of syphilis(OR = 3.17,95% CI:1.25-7.99) was significantly associated with the HIV infection. Conclusion:The unprotected sex behavior and multiple sexual partners are common among MSM in Yangzhou. Furthermore,the infection prevalences of HIV and syphilis were relatively high. Intervention on the behavior of the target population must be strengthened to reduce the HIV spread in such population to general population.