Evolution of microstructures of alloy MlNi_(3.75)Co_(0.75)Mn_(0.3)Al_(0.2) (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) and β 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 (hydriding-)(dehydriding) process. It’s 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. Evolution of microstructures of alloy MlNi_ (3.75) Co_ (0.75) Mn_ (0.3) Al_ (0.2) (Ml-La-rich mischmetal) during the electrochemical hydriding-dehydriding process was studied by using in situ X-ray diffraction method is indicated that both both crystallite sizes of α phase (solid solution phase) and β 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 (hydriding -) (dehydriding) process. It’s also clearly found that strain along (110) plane and (001) plane of a 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.