Problematic internet use (PIU) is a new type of behavioral addiction emerged in this age of information. Furthermore, the boys with problematic internet use are more than the girls. People with problematic internet use can’t properly control their use of internet and eventually harm their psychological, social, academic or work performance. Therefore, detecting the underlying reason of the diversity of PIU between different sex groups has both theoretical and practical implication. The present study intended to investigate how internet usage and personal factors would contribute to the diversity of PIU between different sex groups. Firstly, we detected the characterization of Time 1 internet usage (both time and activities) . Then, utilizing hierarchical regression methods, we explored the relation between Time 1 internet usage and Time 2 PIU: the moderating effect of sex. Thirdly, we investigated the relation between sex and PIU: the mediation effect of effortful control. The main findings were listed below: 1. Adolescents was inclined to long-playing internet use. The activities of internet use focus on chat, game, multimedia events and search. 2. Significant sex difference is not existed in the time but the activities of internet use. The boy is inclined to game, and the girl is inclined to multimedia events, network personal space＆principal sheet, search, e-mail and web teach. 3. Internet time was negatively related to PIU, social and entertainment were positively related to PIU, and information was not related to PIU. The moderating effect of sex is significant in the relation between Time 1 internet time in workdays and Time 2 PIU. The moderating effect of sex was not significant in the relation between Time 1 internet time in weekend and Time 2 PIU, between social activities and Time 2 PIU, between entertainment and Time 2 PIU. 4. Boys’ effortful control is significant lower than that of girls. Significant mediation effect of effortful control is existed in sex difference of PIU. Conclusion: Boys’ effortful control is significant lower than that of girls. Therefore, the boy maybe inclined to play game so as to develop problematic internet use. As boys’ effortful control explained more of the variance of problematic internet use than that of girls.