In this paper, the microstructure and P-C isotherms of AB(5)-type MlNi(3.60)C(0.085)Mn(0.40)Al(0.15) hydride alloy, heat treated at a temperature of 1373 K, with the holding time 2-8 h and subsequently water quenched, have been investigated in detail. XRD results showed that the phase structure of all the samples investigated was CaCu5-type hexagonal structure, whereas the lattice parameters and the FWHM of the alloy were modified by heat treatment. The metallographic microstructure of as-cast alloy was a typical dendritic structure with a number of microcracks at the limb part. After the alloy was heat treated, the microstructure of the alloy was a columnar structure. With the increase in holding time, the grain size increased, the number of the composition segregation dots distributed in the inside of the grain decreased and the width of the grain boundaries broadened. Compared with the P-C isotherms of as-cast alloy, after the alloy is heat treated, the plateau flattened, the plateau pressure decreased and the hysteresis lessened.