The purpose of this experimental research was to study the effect of group counseling based on behavior therapy on physical self - concept and weight reduction of Mathayom Suksa V students of Si Ayuthaya School in Bangkok in the academic year 1993. The subjects of twelve students were randomly selected from overweight students whose physical self - concept scores were lower than the percentile ranks of 50. Then the subjects were randomly divided into an experimental group and a control group, each group was consisted of six students. The experimental group participated in group counseling program based on behavior therapy while the control group was given information on behavioral weight loss treatment and physical self - concept. The data were analyzed by the Wilcoxons matched - pairs signed - ranks test and the Mann - Whitney U Test. The results were as follows : 1. The students who received group counseling based on behavior therapy showed significant increases in physical self - concept (p < .05) and lost significant amounts of weight (p < .05). 2. The students who were given informations on behavioral weight loss treatment and physical self - concept showed significant increases in physical self - concept (p < .05) and lost significant amounts of weight (p < .05). 3. The group counseling based on behavior therapy produced significantly greater increase in students physical self - concepts and reductions in weight than the information treatment (p < .05).