Abstract: Objective To estimate the population size of the men who have sex with men(MSM) in Beijing and to provide references for acquired immunodeficiency syndrome(AIDS) intervention and rational allocation of health resources among MSM in Beijing.Methods With two-stage sampling,a questionnaire survey was carried out among MSM who had participated in activities at Beijing gay venues,human immunodeficiency virus(HIV) voluntary counselling and testing(VCT) clinics or gay websites to obtain relevant data.Three sources capture-mark-recapture method(CMR) was used to estimate the population size of MSM in Beijing.Results The population base of MSM in Beijing was estimated to be 78 283,with a standard deviation of 7 682 and a 95%confidence interval of 63 226-93 340.Conclusion Scientific estimation of cardinal number of MSM in Beijing was made using three sources CMR first in the study.