Abstract: Objective To understand sex related knowledge,attitudes and behaviors among middle school students in Shanghai.Methods A total of 3 069 grade one junior high or senior high middle school students from 11 middle schools in one district of Shanghai were interviewed by using anonymous self-administered questionnaire.Results The knowledge scores of middle and high school students were rather low with the mean score being 33.85 and 55.08 respectively,while their attitudes toward sexual issues were relatively open.About four fifth of the high school students and more than half of the middle school students approved or accepted premarital sexual behav ior.A quarter of the high school students and 6 percent of the middle school students ever dated or had a hug with opposite sex.Conclusion With the increase of age,middle school students were more likely to hold anopenattitude to sex and were more likely to have sex related behaviors.However,they had poor knowledge on reprodctive health.It was necessary to strengthen family and school sex education in Shanghai.