Bone mineral density (BMD) is the amount of bone mineral in bone tissue. BMD measurement is used in clinic as an indirect indicator of osteoporosis and bone fracture risk. Dual-energy x-ray absorptiometry (DXA) is the most widely used technique for BMD measurement. However, there are clinical reports about the reliability of DXA and CT for BMD measurement. This work is to provide a mathematical analysis about the limitation of BMD measurement by x-rays.