This study explored the effect of a social emotional learning programme- the ECE-Peace Ambassador Project (ECE-PAP) − on children's (i) emotional intelligence, (ii) prosocial behaviour, (iii) aggressive behaviour with peers, (iv) and peer exclusion. Employing a quasi-experimental design, ECE-PAP was implemented in 16 schools in Hong Kong. In each school, teachers in one K3 class (4- to 5-year-olds) implemented the programme and a second K3 class served as a control (N=302). Multiple reporters (children, parents, and teachers) provided assessments of children's emotional competence, social behaviour, aggressive behaviour, and responses to peer aggression the week before the programme, the week after, and three months later. Preliminary analyses showed that at pretest, children in the experimental condition showed lower emotional intelligence and less competent responses to peer aggression than controls. However, these differences disappeared by posttest and remained non-significant at follow-up. Children in the experimental condition showed increases from pre- to posttest in emotional intelligence, emotion labelling, and taking action to stop bullying, and decreases in avoidance and aggressive responses. Implementing the programme over a greater number of weeks was associated with more positive child outcomes. These results suggest that ECE-PAP may be an effective, cost-effective way to address aggressive behaviour in kindergarten classrooms by promoting children's emotional intelligence and social skills. Copyright © 2024 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.