Aims: To investigate the online activities, prevalence of Internet Addiction in relation to demographic characteristics and risk factors related to family and school among adolescents. Methods: A total of 6468 10–18year old adolescents recruited from local schools in Guangzhou, China were selected by adopting multi-stage stratified random sampling (female/male: 2886/3582; mean age:13.78±2.43). Participants completed a structured questionnaire. Results: The overall prevalence of Internet Addiction was 26.50%, with severe addiction being 0.96%. Internet Addiction was higher among males than females (30.6% versus 21.2%). Older grade students reported more Internet addiction rate (χ2=431.25, P<0.001). The five highest-ranked online activities were social networking (94.73%), school work (86.53%), entertainment (82.44%), Internet gaming (73.42%) and shopping online (33.67%). A negative relationship with teachers (OR: 1.35, 95% CI: 1.20–1.53), a negative relationship between two parents (OR: 1.23, 95% CI: 1.18–1.37), and poor academic performance (OR: 1.22, 95% CI: 1.17–1.35), showed the highest relative risks for Internet addiction. Conclusions: Severe Internet Addiction is not common, but mild Internet addiction was reported by more than one fourth of all participants. The rates of Internet Addiction varied by gender, grade, the quality of family relationships and school situation, suggesting these factors should be considered when designing and implementing interventions. Keywords: Internet addiction, Adolescents, Social network, Parenting, Relationship