Abstract: Objective To explore the prevalence and the influencing factors of internet addiction disorder (IAD) among middle school students in Shijingshan district of Beijing city and to provide bases for IAD prevention and cessation.Methods Totally 1 252 students were selected from 38 classes with stratified cluster random sampling and investigated with a questionnaire.The prevalence of IAD was analyzed and related factors were assessed.Results Among 1 218 students who effectively answ ered the questionnaire,the rate of internet use was 91.79% (1 118/1 218) and the prevalence rate of IAD was 1.84% (59/1 218).The first six intentions of IAD were playing games on line(62.71%,37/59),listening to music and listening to/watching/downloading movies or television programs(57.63%,34/59),instant messaging(QQ/MSN/POPO) (52.54%,31/59),internet chat romm(30.51%,18/59),receiving and sending e-mail(28.81%,17/59),and uploading and dow n loading files (28.81%,12/59).The main places of surfing net were home (72.88%,43/59) and net bar (20.34%,12/59).The results of logistic regression indicated that the risk factors of IAD were with father not working in administrative institution,long time of internet surfing,long time of playing video game,and with dysthymia symptoms (loneliness,isomnia,and depression) and the protective factor of IAD was female gender.Conclusion The IAD among the middle school students should be concerned and early preventive measures should be taken,especially for major influencing factors of IAD.