The main objective of this research is to investigate the effectiveness of group training, based on transactional analysis affecting the aggressive behaviors of the girl students of grade one in high school, in the city of Rasht. This experimental research was carried out on 40 individuals from those students who were rated “high” in aggression test (AGQ).This sample was selected through the phased random selection and it was divided in two equal groups (experimental and control group),randomly. Subsequently, the group training program was represented to the experiment group, after the method of transactional analysis, based on the assembly of concepts, within 8 ninety-minute sessions for each week , and during this period, no interference was imposed on the control group. Then, after the end of training, the AGQ-test (post-test) was applied to both experiment and control groups simultaneously and under the identical circumstances. The obtained information from the pre-test and post-test were analyzed using the statistical T test for the independent group and MANOVA. The additional obtained results suggested that there was a significant difference between the experimental and control groups (P=0.0001). In other words, the experimental interference (group training based on the transactional analysis) has given rise to reduce the aggression and its components in experimental group.