The purpose of this study was to identify the factors influencing mobile phone dependency in adolescents. Data were obtained from the 3rd wave of Korea Adolescence Welfare Panel Survey 2012. The subjects were 2,162 middle school students. The significant predictors of mobile phone dependency in adolescents were depression, aggression, attention, social withdrawal, and self-identity. These variables explained 14% of the variance in mobile phone dependency. Based on the study findings, mood status such as depression, aggression, attention, social withdrawal, and self-identity should be considered to conduct prevention and management programs of mobile phone dependency for middle school students.