[en] In powder metallurgy, the microstructure of the sintered materials is established during the thermal process. Theoretically, for ideal systems, the grain size evolution could be described mathematically by the grain growth laws. But, the practice shows that in real systems the theoretical laws are not respected, the main reasons being presence of pores, particle size distribution, and for doped systems the additive/liquid volume. The difference between theoretical predictions and experimental data is larger especially in doped oxide systems at very low addition level. Starting from the basic theory of the grain growth in solid and liquid phase sintering, and from the microstructure characterization of oxide doped systems, this work proposes a model of the grain size evolution in the presence of very small liquid fraction. This model considers all possible transfer processes governing the grain growth: grain boundary diffusion, surface reactions and diffusion through the liquid phase. It explicitly takes account of the additive volume and its continuous distribution through the matrix during grain growth process, of the wetting properties, if a liquid phase exists, or of the diffusion parameters change if a solid solution occurs. To quantify these aspects, particle surface fraction on which the additive is distributed at any time in solid or liquid state was introduced. The model validation, done for UO₂-Nb₂O₅ and UO₂-TiO₂ systems, sintered at 1700 deg. C, showed a good agreement between its prediction and the experimental data. (author)