The purpose of this study is to find out whether accumulated parts of body fat coincide with reduced parts after 12 weeks of training, and how age affects body compositions during 12 weeks of aerobic training with 90 subjects ages 30 to 50. To analyze body compositions the Anthropometric Method, Skinfolds Method, and Bioelectrical Impedance Analysis, (BMI Method) ㎏/㎡ were used. The exercise intensity was evaluated by the formula of Karvonen [Target Heart Rate: THR%=(THR%)(HRmax-HRest)+HRest]: the first phase (1st-4th week) was THR 60%, the second phase(5th-8th week) was 70% and the third phase(9th-12th week) was THR 80%. The exercise was done 3-4 times a week for 12 weeks. The 0.05 level of significance was as critical level for the study. As a result of this study, the following conclusions are reached. 1) The weight was reduced at a significant level (p<.05) in the case of 30 and 40 year-olds. In the case 50 year-olds, there was little reduction in their weight. 2) Circumferences of all body parts showed differences after 12 weeks of the exercise training. Circumferences of the hips, the waist and the thighs showed very big differences. In particular, women had a higher rate of differences in circumferences of the hips and the waist. 3) When it comes to skinfolds, a statistically significant reduction(p<.001) in seven parts of the body appeared after 12 week of the exercise training. In the case of men, the thigh showed the highest rate of difference, followed by superior iliac, triceps brachii, the abdomen and when it comes to women, the abdomen changes at the highest level followed by the thigh, superior iliac, triceps brachii. 4) In the case of the Bioelectrical Impedance Analysis, the rate of body fat showed differences at a statistically significant level (p<.001) when the subjects were men in their 30`s and 40`s, but when they were in their 50`s the reduction was dramatically decreased. When it comes to Lean Body Mass, the weight of male subjects reduced by 0.48㎏ and female subjects showed an increase in their muscles by 0.64㎏ after 12 weeks of exercise training. Generally most of the age groups showed an increase in Lean Body Mass. In the index of body fat, men showed a decrease of 0.89 and women 1.44. However, there were no statistically significant differences between the two groups. The percentage of WHR decreased by 0.02 in the case of men, and women had 0.05 of decline (p<.001). In this case all age groups showed a statistically significant level (p<.001), but male subjects in their 40`s had a rather lower significant level (p<.05)