Abstract: Objective To examine syphilis epidemic and its influential factors among female sex workers (FSWs) in Yunnan province.Methods A cross-sectional survey was conducted among 25 574 FSWs selected with probability sampling at 118 county-level FSWs surveillance sentinel sites in 16 prefectures or municipalities of Yunnan province during April through June 2013.The survey included a questionnaire interview on demographics,acquired immunodeficiency syndrome (AIDS)-related knowledge and high-risk behaviors and blood sample detection for syphilis infection.Results The prevalence of syphilis infection was 1.5% among the FSWs surveyed and the FSWs without syphilis infection had a significantly higher awareness rate of AIDS-related knowledge than those with the infection (98.02%24 703/25 203 vs.94.1%349/371)(χ²=28.47,P=0.000).Compared to those with syphilis infection,the FSWs without the infection had significantly higher rates for condom use in the latest commercial sex and consistent condom use in commercial sex during previous one month (χ²=17.28 and 73.89,both P=0.000);the rates of drug use,injection drug use,and needle sharing were significantly higher in the FSWs with syphilis infection than in the FSWs without the infection (χ²=72.38,9.45,and 5.0;P=0.000,0.02,and 0.025,respectively).The results of univariated and multivariate analyses showed that working at low or middle grade entertainment places,migrating from other provinces,with the education of junior high school and below,at the ages of 20 years and above,drug use,and without AIDS-related knowledge were risk factors of syphilis infection and consistent condom use in commercial sex during the previous one month was a protective factor of the infection among the FSWs.Conclusion Interventions on prevention of syphilis infection should be carried out to reduce syphilis epidemic among the FSWs in Yunnan province.