The purpose of this study was to investigate the effectiveness of social skills intervention program via peer tutoring on aggression level of secondary school students. The study was a semi-experimental model using experimental group with pre and post-tests. Eleven (eighth class) students were selected to have skills for being peer helper. The population of this research consisted 56 secondary school students at Kahramanmaraş. In order to evaluate aggression level of students, Aggression Scale developed by Tuzgöl (1998) was used. . Additionally, a Personal Information Sheet developed by the researcher was used to record certain demographic variables. Researcher applied social skills invention program to 11 students (peer helpers) for eight weeks. Later, peer helpers applied intervention program to selected 56 students as well. After application, last-test was applied to selected 56 students, again. To analyze the data collected, SPSS 15 for computer was used. Results of the research revealed that the social skill program via peer helping (peer guidance) considerably decreased the level of aggression of students at secondary school students. Students’ aggression level differentiated according to their sex, but not their class levels. Results were discussed in the light of literature