Abstract: The dry and wet high-energy milling had been applied to prepared AB5 type Ml(NiCoMnAl)5 nano hydrogen storage alloy.X-ray diffraction was used to characterize the structure of the alloy and gas reaction controller was used to perform the hydrogen storage properties test.The effect of nanostructure on the hydrogen storage properties was analyzed.The result show that both the methods could make the grain size of the alloy reduce to less than 20 nm after milling for 4 h,and the decrease of the grain size by wet milling was faster than that by dry milling.Furthermore,the increase of the lattice defect of the alloy by wet milling was less than that by dry milling.The alloy powders after wet milling had significantly less lattice defect than that of after dry milling in the same time.Nanostructure and the increase of lattice defect by high energy ball milling led to the decrease of hydrogen storage capacity,and the slope of pressure-composition-isotherm (PCI) of hydriding and dehydriding pressure platform of the alloy.In addition to grain size,the lattice defect of the alloy had a significant impact on hydrogen storage properties.With the increase of the lattice defect,the hydrogen storage capacity of the alloy reduced.