PURPOSE: Patient age significantly influences the rate of fracture healing. The rate of healing declines with increasing age. The authors compared the aging effect on fracture healing in the callus of rat femur by the light microscopy. MATERIALS AND METHODS: In this study the unilateral, closed fractures were created in the femur of 18 Sprague-Dawley rats. The rats were killed in three age group(8 weeks:7, 32weeks:6, 70weeks:5) at 2 weeks after fracture. The composition of fracture callus(new bone, cartilage, mesenchymal layer) was measured by image analyzer with H-E stain. Immunohistochemical stain (PCNA, TUNEL, TRAP) positive cells were counted for the comparing of cellular activity according to the aging. RESULTS: The percent of intramembranous new bone in the younger rat(8 week:22.32%) was higher than the older ones(30 week:7.09%, 70 week:5.37%). The percent of PCNA positive osteoblast in the newbone decreased according to the aging(8 week:64.25%, 30 week:57.40%, 70 week:29.54%). The number of osteoclast in the osteochondral junction at the 8 week(43) was more than that of 30 week(25.57) and 72 week(29.87). The number of TRAP positive osteoclast was not different as aging, but the number of osteoclast in the osteochondral junction(5.89) was more than that in the metapyseal area(2.08). CONCLUSIONS: More new bone was found in younger rat. There was a strong correlation (p<0.05) between age and PCNA activity. More number of active osteoblast and osteoclast was found in younger rat femoral fracture callus, which indicated rapid fracture healing in younger age.