Ag sinter joint is gaining greater attention for use with wide-bandgap die attach modules in high-temperature applications. In this study, we have investigated both the necking growth of Ag grain and the mechanical properties of sintered Ag with different Ag particle shapes in a low-temperature, pressure-less sintering process in air and N-2. The necking growth ratio of Ag grains plays an important role in the mechanical properties of sintered Ag. Large necking growth corresponds to stronger tensile fracture strength of sintered Ag paste and stronger die shear strength. Self-generated Ag nanoparticles (AgNPs) with of a size less than 10nm were observed under an air atmosphere, an atmosphere that produces larger necking growth than does that of N-2. In addition, self-generated AgNPs were observed in sintered Ag flake particles specimens. Ag flake particles also lead to larger necking growth than that of sintered Ag spherical particle specimens. Self-generated AgNPs accumulated together due to their high surface energy, thus improving the necking growth of Ag grain and leading to a stronger fracture strength. In addition, it was revealed that both the oxygen element and the grain residual strain of initial Ag particles play important roles in the AgNPs self-generation during the sintering process. This study enhances our understanding of the sintering mechanism of Ag particles and the relationship between grain necking growth and its mechanical properties.