Abstract: Objective To examine the prevalence and predictors of unprotected anal sex among men who have sex with men(MSM) in Chongqing municipality, China.Methods Two consecutive cross sectional surveys were conducted among MSM in three districts in June July, 2006 and 2007.Information on demographic characteristics, HIV knowledge, sexual and drug use behavior, HIV/STD related services and self reported symptoms of sexually transmitted diseases(STDs) were collected.Results A total of 1434 MSM were recruited in the two surveys.The average age of the participants was 27.0 year.Of all participants, 22.2% were married, 86.1% received high school or higher level education, 65.7% were self-identified as hmosexual and 31.3% as bisexual.The rate of unprotected anal sex in the previous 6 months was 63.6%.Factors independently associated with unprotected sex included old age (odds ratioOR=1.02, 95% confidence interval CI: 1.0-1.03), having no commercial sex work with another male partner(OR=3.0, 95% CI: 1.9-4.7), receiving no STDs service (OR=1.4, 95% CI: 1.1-1.7), self reported STDs (OR=1.7, 95% CI: 1.2-2.3), and high level of HIV/AIDS knowledge (OR=0.9, 95% CI: 0.8-1.0).Conclusion The prevalence of unprotected anal sex among MSM in Chongqing was very high.Multiple sex partners and heterosexual behaviors were common.HIV and syphilis prevalence had reached a high level.