This study examined the effects of a game program of assertive behavior, helpful behavior, and self-concept in children aged 8 to 10. A pretest-intervention-posttest design was used with a sample of 126 experimental and 28 control subjects. Three instruments were administered before and after intervention: the Children's Assertive Behavior Scale, Sensitivity Scale, and the Adjective Check List. Assertive behavior, helpful behavior, and self-concept were the dependent variables. Intervention consisted of a group play-session carried out once a week during the whole academic year. The program's games encourage children to cooperate and share, as well as to develop creative play. The program included presentation, communication, help, cooperation, creativity, and emotional expression games. Results of multiple variance analyses suggest that the program caused significant improvement in assertive behavior (p < .001) and self-concept (p < .01). This paper reveals empirical evidence of the positive contribution of cooperative-creative play to child development.