Mixture of Ti and Mo powders was mechanically alloyed to obtain β-type Ti-20mass-%Mo alloy. The mechanism of mechanical alloying(MA) of Ti-20mass%Mo and microstructure of its product were investigated by X-ray diffraction, XAFS and TEM. X-ray diffraction profile showed that complete alloying was attained by the 36ks-MA, but incompleteness was revealed by the analysis of XAFS. According to the result of XAFS, 144ks-MA was needed to accomplish alloying. The radial structural parameters of j-th shell, R _j , N _j and σ _j are obtained by curve fitting method on a two-shell, double Mo-Mo and Mo-Ti pairs. Nearest neighbor bond length(R _j ) decreased with the time of MA. From the observation of TEM, it was revealed that mechanically alloyed particle was composed of small domains with different lattice images in direction, like subgrains. Deformation of the lattice image was also recognized for the MA specimen.