A method for calculating the kinetics of nucleation of the Ni3Al phase formed during decomposition of a supersaturated solid solution is considered. The calculations and the experiment prove nucleation of clusters in the incubation period of decomposition of the solid solution upon quenching cooling. Using the developed technique [1], the initial sizes of the clusters are calculated, as well as the nucleation time and time of cluster growth to the average size, and also the time of formation of a given volume fraction of the phase during thermal treatment. The obtained data are used for plotting of isothermal diagrams of decomposition to optimize heat treatment regimes of the alloys.