It has previously been observed that after the third decade of life the tensile strength of bone decreases faster than its density [1], indicating that some microstructural change occurs in bone with advancing age. In order to try to elucidate this observation, the orientation and particle size of the mineral phase of bone have been studied with respect to age. Samples were taken from femora varying in age from 13 to 97 years. The results show that the degree of preferred orientation does not vary with age after the first decade of life. However, the percentage of small particles increased up to the fourth decade of life and thereafter decreased, a pattern of change similar to that observed for bone strength. It would seem to indicate that an increase in mineral particle size is a factor that contributes to the loss of bone strength and that an increased percentage of large particles in the later years of life could, along with osteoporosis, help to explain the increased fragility in the bones of the elderly.