Purpose: This study was done to explore the correlation among mobile phone overuse, mental health and school adaptation, and to identify factors related mobile phone overuse. Methods: A questionnaire was filled out by 943 elementary school students from 3rd to 6th grade in the cities of Busan and Ulsan. Frequency, percentage, mean and standard deviation, t-test, ANOVA, Pearson`s correlation coefficients and stepwise multiple regression with SPSS/WIN 12.0 program were used to analyze the collected data. Results: Mobile phone overuse in the elementary school students was positively correlated with mental health, and negatively correlated with school adaptation. Stepwise multiple regression analysis showed the factors as school grade, gender, depression, anxiety, learning adaptation, adaptation to friends and adaptation rules significantly accounted for 23% of the variance for mobile phone overuse. The most significant factor influencing overuse of mobile phone was school grade. Conclusion: These results suggest that early screening and intervention programs for students with low mental health and low school adjustment would be helpful in preventing future mobile phone overuse.