The rapidly quenched treatment produced significant effects on the electrochemical characteristics of Mm(NiMnAlCu)(4.9)Co-0.2 alloy. It made the activation capability and the discharge capacity decrease, while the cycle stability improved and the degree of the effects increased with increase of the quenching rate. The results of X-ray diffraction analysis showed that the structures of the main phase and secondary phase of Mm(NiMnAlCu)(4.9)Co-0.2 alloy are both of the CaCu5 type, but the lattice constants of two structures have a little difference. The amount of the secondary phase in the as-quenched alloy is very little, and rapidly quenched treatment made the structure of the alloy more homogeneous. The lattice constants of the main phase of the as-quenched alloy are larger than that of the as-cast one. Amorphous and crystal co-exist in the alloy with quenching rate of 15 m/s. The Co content was reduced from 10.0 wt.% to 2.8 wt.% through suitable rapidly quenched treatment, and it is not necessary to add other expensive elements in the alloy. (C) 2003 Elsevier B.V. All rights reserved.