The purpose of this study was to investigate the relationship among internet addiction, impulsiveness and school adjustment of undergraduate students. I examined whether there were differences in the levels of addiction, impulsiveness and school adjustment by gender of the students and whether there were differences in school adjustment and in impulsiveness by gender and addiction types.This study was conducted on 406 undergraduate students: 228 males and 169 females. I used the questionnaire by Oh(2005) in order to measure the level of internet addiction, the questionnaire by Kim, Kang, Choi(1998) to measure the school adjustment, and the questionnaire by Lee(1992) to measure impulsiveness of undergraduate students. The collected data were analyzed with t-test, test and one-way ANOVA using SPSS 11.0 package.The major findings of this study are as follows:First, there were differences in the level of internet addiction by gender. That is, males' level were higher than females in the internet addiction.Second, there were differences in school adjustment by the level of internet addiction. That is, addictive internet users lower than general users in the level of school adjustment. Third, there were differences in impulsiveness by the level of internet addiction. That is, addictive internet users higher than general users in the level of impulsiveness. In summary, the result of this study shows that the addictive internet use have negatively effects on school adjustment and impulsiveness of undergraduate students.