Objective This study is to estimate the population size and prevalence of HIV and herpes simplex virus type 2 (HSV-2) infections among female sex workers (FSWs) in Kaiyuan,Yunnan Province,China.Methods Eight cross-sectional studies were conducted among FSWs in Kaiyuan from 2006 to 2009.Based on the data from two study time points each year,the total numbers of FSWs and HIV positive FSWs were estimated by using the capture-recapture technique (CR).Results Estimated sizes of FSWs in Kaiyuan were 1 629,1 672,1 186,and 1 014 in the respective years from 2006 to 2009.Although the crude prevalence rates of HIV and HSV-2 varied over time,the adjusted prevalence among this population was relatively stable at 10％-12％ and the adjusted HSV-2prevalence ranged from 67％ to72％.Conclusion The reason for the slight decrease of the size of the FSW population is unknown.The adjusted prevalence rates of HIV and HSV-2 among them were stable over the course of this study.