The effect of heat treatment on microstructure, equilibrium hydrogen sorption pressure and plateau slope of LaNi3.8Al0.75Mn0.45 alloy was investigated. X-ray diffraction (XRD) analysis indicates that annealed alloys have single phase and the same hexagonal structure as that of LaNi5 alloy (CaCu5 type, P6/mmm ). The cell parameters of alloys fluctuate with the increasing annealing temperature. The equilibrium hydrogen pressure and plateau slope are a parabola function with annealing temperature for LaNi3.8Al0.75Mn0.45 alloy. By this relationship, an appropriate heat treatment temperature for LaNi3.8Al0.75Mn0.45 alloy is determined to about 1220-1230 K by mathematic simulation process. However, the maximum hydrogen storage capacity of alloys does not affected by the annealing temperature.