The macrokinetic approach to the structure formation under crystallization of a binary system from the melt is developed. A two-phase model (melt and refractory grains) including the kinetic functions of nucleation and growth, heat and mass balances and phase diagram as specific boundary conditions is considered. Calculation of macrostructure is carried out. The influence of kinetics, heat and mass transfer on distribution of grains against their size and macrokinetic features of the process are analyzed.