Development of biocompatible Ti alloys with low Young's modulus (modulus of elasticity) is demanded for orthopaedic implants. Young's modulus of beta phase Ti-Nb binary alloys was measured as a function of composition along with microstructural observations. Young's modulus of solution treated and quenched alloys exhibits a minimum value of 62 GPa for Ti-42wt.%Nb. Transmission electron microscopy indicates that athermal omega transformation is induced in unstable beta alloys, and the minimum in Young's modulus appears at a composition where athermal omega transformation is suppressed by quenching, as alloying content is increased. Based on the obtained results on the binary alloys, ternary additions to Ti-Nb were investigated to further decrease Young's modulus. It is found that Sn and O additions suppress omega transformation and Young's modulus of solution treated and quenched alloys is decreased to about 50 GPa by optimizing compositions in Ti-Nb-Sn and Ti-Nb-O alloys. Composition dependence of Young's modulus will be discussed on basis of the theoretical DV-Xa method.