In recent years, the power device industry has developed at a high speed. The development trend of high power and high density of power chips has put forward higher requirements on the service temperature and connection strength of power chips.1-2 Nanoscale metal particles, as a material bonded at low temperature and served at high temperature, had high electrical conductivity, good fatigue resistance, and excellent corrosion resistance.3-4 Therefore, the sintering performance of NPs has received extensive attention in the field of microelectronic packaging. Previous experimental and theoretical studies have focused on thermal sintering, 5-8 but most flexible substrates could not serve at high temperature, and for this reason, pressure-assisted sintering technology has been proposed. The study of atomic motion and sintering mechanism of NPs at the microscopic scale could improve the performance of NPs and optimize their applications. In this paper, a molecular dynamics (MD) approach was used to simulate the process of pressure-assisted sintering of two Ag NPs of different sizes at 450K. Tensile simulations were conducted with the sintered structure to evaluate the necking strength. The sintering mechanism and strength of NPs of different sizes sintered under different pressures were analyzed. It concluded that, for the pressureless sintering or low-pressure sintering, size differentiation design for the particles pair improved the sintering performance and interconnection strength. While the improvement was found to be greatly reduced when the sintering pressure was increased. Further research reveals a transformation of material transport mechanism dependent upon the combining of pressures and sizes of the sintering process. Increasing the sintering pressure to a certain threshold will induce the transport mechanism of material to transform from surface diffusion to plastic deformation.