To study the relationship between the bone mineral density measured by DEXA and QCT methods and to study the factors influencing on the value, bone mineral density in 208 patients with low back pain were measured by the two methods and compared statistically. Ages were varied between 17 to 79(Av. 46.7) years and male and female were 86 and 122. The results of this are as follows; 1. The values of DEXA and QCT showed statistically significant relationship(γ=0.58) as a whole. And bone mineral density in L2, L3 and L4 showed no difference in both groups. 2. Bone loss by aging(per year) in male and female was 0.99% and 1.06% by DEXA, 0,99% and 1.41% by QCT. 3. Body height and weight showed no significant influence of the value in two methods. 4. In body mass index over 27.5, the value measured by QCT was influenced to be diminished while that of DEXA was not influenced. 5. The value by QCT was influenced to be diminished by the increase of age especially over 50 years old in both sex. While the value by DEXA was influenced to be lowered than expected in female over 50 years old. It is concluded that the methods of measurement of bone mineral density have their limitation depend on the age, sex and mass index.