PURPOSE: This study was done to identify the level of Internet addiction among adolescents, and to examine factors affecting Internet addiction in adolescents. METHOD: The participants in were 475 high school students. Data were analyzed using multiple regression analysis to examine the factors affecting Internet addiction in adolescents. RESULT: The overall Internet addiction scale score was 63.83(+/-20.48). The differences of Internet addiction according to student characteristics were found to be significant for sex and grade. The level of Internet addiction was positively related to the level of impulsiveness and depression. The level of Internet addiction was negatively related to the level of social support. Factors such as impulsiveness and social support were examined as significant factors predicting Internet addiction in adolescents. CONCLUSION: Based upon these findings, factors affecting Internet addiction in adolescents include impulsiveness and social support. The findings provide further justification for interventions targeting key factors that influence Internet addiction in adolescents in addition to providing adolescents with necessary communication, negotiation and other life skills.