B-site cation order-disorder transition induced by mechanical activation was observed in Pb(Mg1/3Nb2/3)O-3-Pb(Mg1/2W1/2)O-3 (PMN-PMW) solid solution, which was examined using both XRD diffraction and Raman spectroscopic study. The order-disorder transition is composition dependent. Mechanical activation triggers the B-site disordering, which can be steadily recovered by thermal annealing at elevated temperature, i.e., at temperatures around 600degreesC. Raman spectroscopy demonstrated that there existed tiny ordered microdomains in 0.4PMN-0.6PMW subjected to up to 20 h of mechanical activation, although they cannot be shown by X-ray diffraction. This is a result of the equilibrium between the mechanical destruction and temperature-facilitated recovering at the collision points during mechanical activation. It is therefore unlikely that a complete disordering can be realized in PMN-PMW by mechanical activation. The disordering in PMN-PMW triggered by mechanical activation occurs simultaneously with the refinement in crystallite size at the initial stage of mechanical activation, suggesting that the fragmentation of crystallites is responsible for the order-disorder transition at least during the initial stage of mechanical activation.