We report on advanced beta-titanium alloys having excellent biocompatibility without containing V or Al that has been known as a toxic element for human body, and on a low elastic modulus to be used in the fields of artificial joint and dental implant in the replacement of natural human bone. Martensite structure with alpha' (hcp) or alpha" (orthorhombic) was observed in Ti-xNb-1.5at.%Si, where x=10-20at.%. The crystal structure of martensite formed from water quenching process was largely dependent upon Nb content but does not on Si content. On the basis of experimental results obtained, it is suggested that Si has an effective role to suppress the precipitation of omega phase leading to reduction in elastic modulus in the metastable beta phase region. Metastable beta phase region was superior to reduce the elastic modulus than stable beta phase region in the present alloy system. The minimum value of elastic modulus was measured to 48GPa.