A simulation method of initial-stage sintering was proposed to understand the effect of sintering conditions on the sintering behavior of two particles of different size and composition. In this study, This study employed a homogeneous solid solubility system of Cu Ni. The sintering experiments were performed with two spherical Cu-Ni particles in contact, and the sintered particles were observed by SEM and EDX, It was confirmed that the sintered particles always form a curved neck and copper was the dominant component in the neck. The proposed simulation method, in which the model for two particles of identical composition was modified, involves several mass transport mechanisms: surface diffusion, volume diffusion, grain boundarydiffusion-evaporation-condensation and newly, grain growth. The sintering behavior of Cu Ni particles was simulated using the proposed method and the result., agreed well with experimental ones. It was also found that grain growth was not negligible in initial stage sintering, and that shrinkage and grain growth responded sensitively to sintering conditions, Consequently by analysis of these simulation results, it is possible to understand the mechanism of sintering of two spherical particles of different composition and to determine the best sintering conditions.