Abstract: The three-dimensional finite element model of unequal-sized particles system was established based on the generalized thermoelastic diffusion, and the distributions of temperature field and concentration field were calculated to investigate the effect of migration driving force in the preliminary stage of pulse electric current sintering in the unequal-sized particles system. In the results, the vacancy concentration gradient, the temperature gradient, and the concentration gradient caused by the changes in temperature and stress are the common driving forces of material migration in particle neck. The numerical results show that, there are two sudden changes of temperatures in particle neck, and the small particles are kept at high temperature during sintering. The change of temperature causes the change of concentration, resulting the higher concentration in neck than in the edge of particles. The thermal diffusion is 2/3 of the total diffusion flux, and the concentration diffusion flux is 1/3, which means the thermal diffusion and concentration diffusion flux are the dominant driving force in pulse electric current sintering process. The driving force of unequal-sized particles is much larger than that of equal-sized particles, providing a theoretical basis for the faster sintering rate of unequal-sized particles in sintering experiments.