Abstract: Objective To investigate the tendency and influence factors of internet addiction (IA) among middle school students in Guiyang city of Guizhou province.Methods A questionnaire survey was conducted for investigation of IA among 3 571 middle school students in Guiyang city.Results The detection rate of IA among the students was 16.9% and the rate of the males(22.1%) was high than that of females(11.7%),with a statistical significance( χ²=55.338,P=0.000).The IA detection rate was positively correlated to the years and frequency of surfing on internet and the total time of internet surfing each time.The IA detection rate was 23.1% for the students with internet surfing history more than 5 years,32.8% for the students surfing on internet more than one time per day,and 48.5% for the students using more than 9 hours each time on internet.The IA detection rate was 25.1% and 23.5% for the students surfing on internet in internet bar and playing online games.The results of multivariate analyses demonstrated that the risk factors of IA were surfing on internet in internet bar frequently,playing online game,surfing on internet more than one time per week,and surfing on internet more than one hour each time,while surfing on internet for getting information was a protective factor of IA.Conclusion It is necessary to guide the middle school students to improve their behaviors related to internet usage for the prevention of internet addiction.