This study aims to analyse the effects of conflict resolution and peer mediation (CRPM) training on the levels of aggression of 10-11-year-old Turkish primary school students. The study was conducted using a quasi-experimental design. The experimental group of the study included 347 students (173 girls, 174 boys), and the control group had 328 students (158 girls, 170 boys). Data for the research were collected using the "Aggression Scale". Data analyses revealed that CRPM training was effective in lowering the aggression scale scores of the students. Boys seem to have benefited more from the training than girls. CRPM training was found to reduce student aggression which may lead to more constructive, restorative and peaceful behaviour in classroom and school settings. Since less energy and time will have to be allocated to resolving problem behaviour, academic success may increase, and a safer school atmosphere may be attained. (Contains 6 tables.)