Abstract: Objective To study the proportion of commercial sex among men who have sex with men(MSM) in Beijing and to provide basis for corresponding AIDS prevention and control program.Methods With stratified random sampling,Simmons model and the corresponding formulas were adopted to investigate the proportion of commercial sex among MSM in Beijing during July to December,2009.Results During the period,the estimated proportion of commercial sex among MSM in actual venues and internet in Beijing were 11.9% and 3.29%,respectively,and the overall rate was 5.11% with the 95% confidence limit of 0.017 to 0.085.The proportion of commercial sex event in actual venues was significantly higher than that finding partner through internet.Conclusion The proportion of finding commercial sex partner among MSM in actual venues is higher than that through the internet among the MSM in Beiing.