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The grain boundary diffusion and segregation can influence the grain growth kinetics, the grain size distribu- tion, and therefore the mechanical properties of the ceramic matrix composites. The present paper proposes a phase-field modeling approach to simulate the grain growth in polycrystalline alumina fibers embedded in alu- mina matrix at temperatures above 1000 °C in presence of the grain boundary diffusion of dopants from the ma- trix to the fiber and vice versa. The multi-phase-field model for grain growth [I. Steinbach and F. Pezzolla, Phys. D, 134 (1999) 385] is extended by the incorporation of the grain boundary diffusion, grain boundary segregation model, and the dependence of the interface mobility on the segregation concentration. The kinetic parameters of the model which allow describing the real microstructure evolution were estimated by the comparison to the experimental measurements.The simulation and experimental results of the grain growth with the diffusion of dopants in Nextel 610 fibers show the significant effect of the grain boundary diffusion on the grain size distri- bution. The results of numerical tests were used to adjust the values of the grain boundary diffusion coefficients by the experimental data at different temperatures by means of an inverse method. From the simulations, the diffusion coefficient of Mg was estimated to be 6–7 times higher than that of Si.