considering that mother - child interaction has an important role in child development or disorder,the aim of this study was to investigate the effectiveness of family-based play therapy on improvement of mother - child interaction and decrease of aggression in preschool children. In a Quasi-experimental study with pre-post test, 27 children with symptoms of aggression were randomly assigned into experimental and control groups. Subjects of intervention groups received DIR/Floor time intervention and control group received education about safety on streets. mother-child interaction questionnaire and aggression questionnaire were applied. Data was analyzed using ANCOVA. Findings showed that family-based play therapy, increased mother- child interaction significantly. In the domains of interaction, dependency and conflict decreased significantly but closeness didn’t increase significantly. In addition, aggression in experiment group was significantly reduced compared with the control group.Interventions can effect on mother - child interaction improvement and aggression decrease in Children. It shows the necessity of comprehensive intervention in relation to variety of developmental challenges of childhood