This study investigated how traditional play by group activities affected young children s prosocial behavior. Participants for the study were 40 five-year-old children attending two kindergartens attached to public elementary schools in Dangjin, Chungnam. Among them, 20 children (twelve boys and eight girls) were assigned to the experimental group while 20 children (eleven boys and nine girls) were assigned to the control group. The instrument used in this study was Prosocial Behavior Instrument for Young Children (Kim, 2003) which is based upon the scale developed by Babcock, Hartle, and Lamme (1995). This instrument consists of 42 items measuring seven factors; leadership, helping, communication, initiative consideration, approaching, sharing, and empathy and self-control. For seven weeks, seven group traditional play including tug-of-war, Ganggangsullae, long-rope play, and counter play had been administered to the experimental group twice a week. The experimental group had been treated with group traditional play activities, while the control group had been treated with general kindergarten curriculum based upon life themes. Collected data were analysed with t-tests in order to find out group differences according to research questions. Results of this study were as follows. Group traditional play activities made statistically significant group differences of young children s prosocial behavior (p<.001). This means that group traditional play activities had positive effects on young children s prosocial behavior.