Abstract: Objective To examine knowledge,attitude and practice about sex among middle school students of Yi minority.Methods All students from two middle schools in a county with human immunodeficiency virus(HIV) infection epidemic in Liangshan Yi Autonomous Prefecture were selected,and a self-administered anonymous questionnaire survey was conducted.Results Among the 1 699 Yi minority students recruited,53.4%(907)knew the three routes of HIV transmission;70.6%(1 200)and 56.1%(953)knew condom and contraceptive being effective contraception measures;25.8%(439)knew that contraception measures could not prevent being pregnant absolutely;59.1%(1 004)were not against premarital intercourse;36.5%(620)reported the experience of masturbated;28.7%(487)reported the experience of having a love affair;and 10.9%(185)reported having sexual behavior,with a higher reporting rate among the boys than among the girls(P<0.01).Over a half of students reported getting knowledge about sex in the school.Conclusion The middle school students of Yi minority are lack of knowledge about sex but they have relative open attitude towards sex and are active on sexual behavior.The results suggest the education on sex knowledge should be promoted among the students.