The cause of a low Young's modulus was investigated in quaternary b-type Ti-Nb-Ta-Zr alloys, as the modulus is decreased to prevent bone absorption and degradation of bone quality when these alloys are implanted into human bones. This investigation was carried out using the alloys 0 single crystals. Acoustic measurements and analysis by the Hill approximation revealed that a low Young's modulus in a polycrystalline form is caused by the low shear modulus c 0 , related to the low b-phase stability, low c 44 , and relatively low bulk modulus B compared with those of binary Ti-based alloys. Furthermore, it was found that the single crystals had strong orientation dependence on Young's modulus, where that in the h1 0 0i-direction E 100 is the lowest of all crystallographic orientations. For quaternary Ti-29Nb-13Ta-4.6Zr alloy (mass%), E 100 is only $35 GPa, which is similar to Young's modulus of human cortical bones as a result of the low B and c 0 . These results indicate that decreases in c 0 , c 44 and B are essential for decreasing Young's modulus of novel b-type Ti alloys which are expected to be developed in the near future.