Evolution of microstructures of alloy MlNi3.75 Co0.75 Mn0.3Al0.2 (Ml-La-rich mischmetal) during the electrochemical hydriding-dehydriding process was studied by using in-situ X-ray diffraction method.It is indicated that both the crystallite sizes of α phase (solid solution phase) andβ phase (hydride phase) decrease with the hydrogen content increasing during this process.β phase is found to be composed of the plate-like and the needle-like crystallites,while the crystallite size of the former changes more rapidly than that of the later during the hydridingdehydriding process.Its also clearly found that strains along (110) plane and (001) plane of α phase and β phase are higher in the α+β region than those in the corresponding single phase region.Based on the microstructural studies,mechanism for the hydride growth has also been proposed.