This articel delivers the experimental results of physical activities played by 4th grade elementary students during physical education (P.E) class. This research is focused on how a systemativally designed phay activity program will affect the development of social mature on elementary students during P.E. free time, and morning study hours. Objectives of this research are to: A. Analyze development of social nature on mid-grade elementary students through play activity. B. Analyze development of subordinate factors of social nature(responsibility, activeness, sociability, emotionality ) on mid-grade elementary students through phay activity. The experiment was condected among 80 fourth graders of \'I\' elementary school in Ichon, Kyonggi Province from September 6th , 2004 to November 27th, 2004 . The students were divided into two groupsL experimental group and controlled group. A play activity program developed by the author was carried out to the experimental group for a period of 12week (24 hours). A normal lesson based on the teaching guidebook was given to the controlled group. A pre- arranged and post- arranged controlled group planning method was used with play activity as independent cariable, and social nature as dependent variable. the main variable of the program and the questionnaire usedin this experiment is the four subordinate factors of social nature-responsibility , avtiveness, sociability, and emotionality. Also, fifteen play programs were chosen to go over pre-examination (September 2th , 2004) experimental step (September 6th ~November 27th, 2004), post-examination (November 2th , 2004) experimental step (September 6th~ November 27th , 2004), post - examination (November 30th, 2004) and statistical analysis. The results are as follows: A. Abalyze development of social nature on mid-grade elementary students through play activity. As for the controlled group, social nature value has risen from an average of 31.52 to 32.48 ( t value= 0.44, p>.05) This shows that a child\' s social nature develops grdually when unaffected. Hoever , when the play activity program was applied to the experimental gorup, the social nature value had significalty increased from 33.07 ( t value = -2.04, p<.05) on average It is well accepted that the development of social nature on elementary students is influenced by play activity program. B. Analyze development of subordinate factors of social nature (responsibility, avriveness, socialbility, emotionality ) on mid-grade elementary students through play activity. After the program was carried out, the experimental group showed a higher average value in activeness ( t value = 3.68, p<.001) soriability ( t value = 3.09, p<.01) and total social nature value ( t value = 2.09, p<.01), while responsibility , and emotionality value showed a rather narrow gap. This shows that continuous guide through play activity program can be linked to development of social nature, The results indicate that this program can help students manage a good personal relationship by cultivating activeness, socialbility, and so on.