In this study, we investigated the microstructure, hardness, Young's modulus, and tensile behavior of binary Ti 4～9 mass Mo alloys, quenched from a temperature of 1223 K. Among the solution treated binary Ti Mo alloys, the lowest Young's modulus was observed in the case of the Ti 6Mo alloy, which was the border composition of a′ /a″ . When the rolling deformation of 2 reduction was carried out, the Young's modulus of Ti Mo alloys having less than 6 mass Mo increased, whereas that of higher than 6 mass Mo reduced. The Young's modulus of the 6Mo alloy hardly changed with the rolling deformation. Ti 6Mo X (Y) alloys, where X and Y are 1 mass of Al, Sn, Cr, and Fe, were prepared in order to investigate the influence of additional elements on the microstructure and mechanical properties of the 6Mo alloy. The phase constitution of quenched Ti 6Mo X Y alloys mostly corresponded with the value of the Mo equivalency (Moeq); however, Ti 6Mo 1Al 1Fe (MAF) and Ti 6Mo 1Fe (MF) exhibited slightly more b rich structures for their respective Moeq values. The lattice parameters``a'' and``b'' of the a″  structure in the Ti 6Mo X Y alloys changed with Moeq in a manner similar to that in the Ti Mo alloy; however,``c'' exhibited a different behavior. The mechanical properties of Ti 6Mo X Y alloys except for the MF alloy were similar to those of Ti Mo alloys corresponding Moeq. However, the Young's modulus of the Ti 6Mo X Y alloys was greater than that of the 6Mo alloy. The tensile properties of the MF alloy having b＋v structure were extremely unstable with respect to the fracture elongation. This peculiar behavior is attributed to the occurrence of deformation induced b＋v→a″transformation. The formation of the a″structure caused significant softening and local intense deformation in the a″phase resulted in a brittle fracture. On the other hand, a successive a″formation induced by work hardening would result in good ductility. It was suggested that the unstable elongation in the MF alloy resulted from competition between the reverse effects.