The objective of this study was to investigate the magnitude of common forms of aggressive behavior and contributing factors observed among Addis Ketema Preparatory School students. Both qualitative and quantitative data were used to conduct the study. Convergent parallel mixed method design was selected. The respondents were 13 teachers, 2 principals and vice principals, 1 guidance and counselor and 309 students. The student respondents were selected using stratified random sampling technique while the principals, teacher and Guidance and counselor were selected using purposive sampling technique. Questionnaire that contained items on socio-demographic variables, scales for aggression and contributing factors were used to collect information from the students. Qualitative data were collected from teachers, Principals and vice principals and guidance and counselor through interview. Percentage, Frequency, mean, standard deviation and independent sample- test and multiple linear regression analyze were conducted to analyses the data.rnThe findings showed that the common of common forms of aggressive behavior in order of magnitude from the highest to lowest are verbal aggression, anger aggression, indirect aggression, physical aggression and hostility aggression respectively. Boys have higher physical aggression level than girls. To the contrary girls have high verbal and indirect aggression behavior, but there is no significant difference between boys and girls in anger aggression and hostility aggression behavior. The Contributing factors were found to be (1) school environment (2) family related factors and (3) family related factors. The school related factors were important predictors of aggressive behavior. Recommendations were also forwarded in the light of the findings.