Bullying behavior is a negative conducted in over and over by some students or more that was attack because of the power an imbalance between the parties involved. In Indonesia, the research of Sejiwa in 2008 for about 1.200 students in Jakarta, Yogyakarta, and Surabaya show the incident bullying in the middle of 66,1%. Sarwono (2013) said that one of the factors that influence the behavior of children toward the bullying behavior is hang out with peer group who deviated from the group that got the admission. From that incident arised an attachment, so peer group attachment became important. The purpose of this research was to know the correlation of peer group with bullying behavior in JHS 2 Gamping. This study used quantitative correlational research with cross-sectional method. The population in this research used adolescent class VII and VIII that number of 399. The sample used stratified random sampling with number of sample 204 adolescent and the instrument of research used questionnaire. The analysis data in this research used the Spearman’s rho. The result as many as 147 adolescent (72,1%) has low peer group attachment, and adolescent who have severe bullying behavior is 133 adolescent (65,2%). The result of correlation is -0,167, and p value 0,017 (p<0,05), it means that the correlations is negative. Based on the result above showed there is a correlation between peer group attachment with bullying behavior in JHS 2 Gamping.