In this study, the structural relationship between negative peer relationships, smartphone dependency, aggression, and cyber delinquency was investigated while the multiple mediation effects of smartphone dependency and aggression in the relationship between negative peer relationships and cyber delinquency was verified. 2,404 middle school 2nd grade students were included for the study and all analyses were performed using jamovi and jAMM modules. The results of this study are as follows. First, negative peer relationships, smartphone dependency, aggression, and cyber delinquency all showed positive correlation with each variable. Second, negative peer relationships had a positive effect on smartphone dependency, aggression, and cyber delinquency respectively. Third, smartphone dependency and aggression were mediated in the relationship between the negative peer relationship and cyber delinquency. This means that students with negative peer relationships tend to have a higher dependency on smartphone use, and accordingly, the degree of cyber delinquency is intensified by aggression increased from the over-dependency on a smartphone. This study proves the mechanism by which students who are sensitive to peer relationships fall into cyber delinquency, and it is expected to be helpful for cyber delinquency-related counseling.