The results of calculation of parameters of the microstructure formed under different conditions of crystallization of multicomponent aluminum alloys are shown, obtained by the numerical model. The model is based on the system the relationship of heat, diffusion and crystallization processes and considers the results of thermodynamic modeling of phase transformations. Fragment of the liquidus surface of the system Al-Cu-Mg built, identifies areas of formation of various structural components and influence on their composition of components under equilibrium conditions and complete suppression of diffusion in the solid phase. The influence of the composition of the alloys Al-Cu-Mg and conditions of crystallization on the kinetics of formation, quantity and parameters of structural components was investigated. The adequacy of the proposed numerical model to predict the microstructure parameters was shown by comparison of experimental and calculated data at different cooling rate and composition of aluminum alloys.