Abstract: ObjectiveTo estimate the population size of men who have sex with men (MSM) in Changsha city and to provide a basis for predicting the trend and adjusting control strategy for human immunodeficiency virus (HIV) epidemic.MethodsA survey was conducted between February and March 2015 in some music bars,public baths,and tea rooms frequently visited by some MSM for their gay activities in Changsha city.Multiplier method and mobile phone MSM dating application (App) were used for the estimation of MSM number.The number (N) of MSM is estimated with a formula:N=m*r;where,r is the average person-times of loging in BLUED App at Changsha region within a week and m is a reciprocal of the ratio between the number of MSM loging in BLUED App within a specific period and the total number of the MSM surveyed.ResultsThe population size of MSM in Changsha city was 46 157 (95%confidence interval:43 854-48 460)，accounting for 1.8%of local male population aged from 15 to 59 years.ConclusionApplying mobile phone dating App to estimate population size of MSM is helpful to obtain more reliable estimation.