We investigated the relationship between body weight reduction and intensity of daily physical activities assessed with 3-dimensional accelerometer during a 3-month exercise program. Twenty-six middle-aged women (58.1±7.4 yrs.) participated in this study. Participants underwent a 90-min endurance exercise intervention once a week, and a 3-dimensionsal accelerometer was attached through the program. Body weight (BW), body mass index, fat mass and percent body fat were reduced significantly (p<0.001), while diastolic blood pressure (p<0.01) and shuttle stamina walking test (p<0.05) improved significantly after the exercise intervention. Moreover, a significant negative correlation was observed between the changes in BW and total activity time (TAT) per day of more than 3METs (TAT≥3METs)(r=−0.580, p<0.01) and TAT≥4METs (r=−0.627, p<0.001). To the contrary, the daily steps and the TAT≥2METs were not related to the changes in BW. After adjusting daily steps, TAT≥3METs (β=−0.630, p<0.01) and TAT≥4METs (β=−0.659, p<0.01) were still significantly related to the changes in BW. These results indicate that weight reduction has a much closer relationship with exercise intensity than daily steps. It is important to keep both quantity and intensity of exercise in the unsupervised exercise program aimed at weight reduction.