Background and Aim: Various studies indicate that increasing and complicating use of cell phones in all age groups and in both sexes is associated with aggression. Despite the widespread use of mobile phones in Iran, psychological and behavioral effects of addiction to it and the consequences have not been investigated yet. The present study aimed at determining prevalence of mobile dependency and its relationship with aggression during adolescence in Birjand in 2011. Materials and Methods: In this descriptive-analytical study, 637 students were selected through randomized double-stage cluster sampling. In order to gather relevant data ,two questionnaires including Hyun young Koo 2009 cell phone dependence questionnaire and Perry aggression questionnaire, which dealt with 4 aspects, i.e. physical, verbal, furious, and antagonistic aggression were used. The obtained data was analysed by SPSS software (V: 15) using X2, independent T, variance analysis, and Pearson's correlation coefficient statistical tests at the significant level P=0.05. Results: Out of 637 student's aged15-20 years, 95.3% (611cases) were moderately dependent on mobile using, 3/4% (22 subjects) ere acutely dependent, and 1.2% (8 students) were addicted to cell phone using. There was no Significant difference between the intensity of dependence on cell phone and age , sex, education, number of phone calls and daily received and sent messages .It was found that there was a corresponding relationship between mobile dependence on mobile association to aggression. Conclusion: According to the present study mobile dependence and aggression in adolescents is less than that in some other studies yet, it is necessary to make interventions to prevent such problems.