La0.78Mg0.22Ni3.48Co0.22Cu0.12 alloy is one kind of nonstoichiometric AB(3.5) type hydrogen storage alloy with low cost and high capacity. In this paper, the effect of annealing treatment on the structure and hydrogen absorption-desorption characteristics of the alloy is discussed. The annealing temperature is determined by using thermogravimetry-differential scanning calorimetry (TG-DSC) tests. The structure of as-cast and annealed alloys is examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD results show that the crystal cell volume decreases after annealing treatment. SEM tests reveal that the structure of the annealed alloys is more regular than that of the as-cast alloy. Influences of annealing treatment on maximum and reversible hydrogen storage capacity, hysteresis factor (H-f) between hydrogen absorption and desorption, sloping factor (S-f), enthalpy changes (Delta H) and entropy changes (Delta S) of hydrogen absorption are discussed in detail. (C) 2011 Elsevier B.V. All rights reserved.