Objectives: The purpose of this study was to identify the degree of internet addiction and factors affecting internet addiction in elementary school students. Methods: A cross-sectional survey design was employed in this study. The subjects were 378 students who were recruited from 5th and 6th grade in five elementary schools in J city, Korea. Data were collected through a structured questionnaire. The data were analyzed using the SPSS Win 10.1 program. Results: Of the children, 51.9% reported being average online users, 45.5%, heavy online users, and 2.6%, internet addicted. The level of internet addiction of subjects correlated significantly with the behavioral activation system, behavioral inhibition system, family cohesion, and learned self-concept, but not family adaptability. Significant predictors influencing internet addiction were the behavioral activation system, learned self-concept, and family cohesion. These predictors accounted for 22% of variance in internet addiction. Conclusion: This study found that the behavioral activation system in personality aspects, family cohesion in the family system, and learned self-concept are primary factors that explain internet addiction among elementary students. We suggest these results be used to develop an online addiction prevention or treatment program.