Purpose: Decreased mechanical stress causes disuse bone atrophy characterized by reduced bone mass and weakened bone. However, few studies have measured the mechanism behind such changes in different areas of bone. The present morphological study investigated the effects of decreased mechanical stress on bone mineral density in different areas of the femur by measuring bone mineral density and assessing the microstructure of osteoblasts. Methods: Twenty-one 9-week-old male Sprague-Dawley rats were acclimatized for one week, and then were divided into control, exercise, and cast immobilization groups. The study was conducted over an 8-week period, from age 10 weeks to 17 weeks in the rats. Bone mineral density was measured by dual energy X-ray absorption (QDR-2000) in the proximal epiphysis/metaphysis, diaphysis, and distal epiphysis/metaphysis of the femur. The microstructure of osseous cells was examined by Scanning Electron Microscopy (SEM). Results: Bone mineral density of the distal epiphysis/metaphysis was significantly lower in the cast immobilization group than in the exercise or control groups (p<0.001). In the cast immobilization group, scanning electron microscopy of the distal epiphysis/metaphysis revealed no boundary between small osteoblast and smooth neighboring cells; however, in the exercise group, actively osteoblastic osteoblasts covered the bone surface, resembling a stone wall.Conclusion: Decreased mechanical stress caused a decrease in bone mineral density that varied in different areas of the femur. The largest decrease in density occurred in the distal epiphysis/metaphysis. Osteoblast microstructure played an important role.