To estimate the prevalence of HIV and syphilis and to assess the predictors of unprotected anal intercourse (UAI) among men who have sex with men (MSM) in Beijing, a community-based survey recruited MSM in 2005 through internet advertisement, community outreach, and peer referral. Demographic, sexual, and HIV risk behavioral information were collected. Serospecimens were tested for HIV and syphilis infections. Of the 526 participants, 3.2% were HIV-positive, 11.2% syphilis-positive, 50% and 43.3% had UAI with regular and casual sex partners, respectively. Participants practicing UAI with regular male partners were independently associated with lower monthly income (adjusted odds ratio-AOR, 1.7; 95% CI, 1.0-3.0) and encountering male sex partners at bathhouses, public washrooms, and parks (AOR, 2.2; 95% CI, 1.0-4.9). Participants practicing UAI with casual male partners were associated with encountering male sex partners at bathhouses, publics washrooms, and park (AOR, 3.0; 95% CI, 1.8-5.2) and more male sex partners having receptive anal intercourse (AOR, 1.8; 95% CI, 1.1-2.9), and was inversely associated with receiving money for sex with men (AOR, 0.3; 95% CI, 0.2-0.7). Professional male sex workers were less likely to practice UAI in Beijing, suggesting the benefits of educational outreach to date. Further education, condom promotion, and prevention of sexually transmitted infections should be intensified urgently to combat the rising HIV epidemic among MSM in Beijing.