The main factors responsible for the kinetics of isothermal bainitic transformation in alloy steels are determined using computer simulation of the solid state phase transformation. The effect of the initial configuration of nuclei of the new phase in the volume of the metal on the parameters of the process of bainitic transformation is determined. Comparative analysis of the computed kinetics of phase transformation at different rates of nucleation of the new phase and the experimentally observed bainite kinetics in steels 25G2S2N2MA, 300M and 50KhMFA is performed.