In orthopedic surgery, bone cement (PMMA:polymethylmethacrylate) has been used for filler of bone defect, stabilizer for internal fixation and depressant for tumor. But few experimental studies were performed with PMMA to determine its effectiveness in relation to the strength by filling bone defect of various size. These studies were performed using paired chicken tibiae to determine the effectiveness of filling various sized bone defects of the tibiae with PMMA. 80 paired tibiae were divided into 4 groups. In group I, one of the paired tibiae was prepared with a 3mm round defect (40% of the diameter) in the cortex, while the other was left intact. In gruop II, one was prepared with a 5mm round defect (66% of the diameter), while the other was left intact. In gruop III, the paired tibiae were prepared with 3mm defect and filled with PMMA in one side. In group IV, the paired tibiae were prepared with 5mm defect and filled with PMMA in one side. Then the tibiae were tested in axial loading until failure. When comparing intact bone to unfilled, a significant decrease in axial load strength of 49.1±5.3 %(3mm defect), 37.4±3.7%(5mm defect) in relation to the defect size (P<0.05). The axial load strength of bone filled with PMMA was decreased also, but the effectiveness of PMMA on bone defect was 40%, which was not statistically significant according to the defect size. It seems that axiall load strength of bone decreases as the size of defect increases, whether filled with PMMA or not, but the effectiveness of PMMA is rather constant.