Molecular dynamics (MD) simulations were employed to investigate the mechanism and kinetics of the solid-state sintering of two crystalline gold nanoparticles (4.4-10.0 nm) induced by low energy laser heating. At low temperature (300 K), sintering can occur between two bare nanoparticles by elastic and plastic deformation driven by strong local potential gradients. This initial neck growth occurs very fast (< 150 ps), and is therefore essentially insensitive to laser irradiation. This paper focuses on the subsequent longer time scale intermediate neck growth process induced by laser heating. The classical diffusion based neck growth model is modified to predict the time resolved neck growth during continuous heating with the diffusion coefficients and surface tension extracted from MD simulation. The diffusion model underestimates the neck growth rate for smaller particles (5.4 nm) while satisfactory agreement is obtained for larger particles (10 nm). The deviation is due to the ultrafine size effect for particles below 10 nm. Various possible mechanisms were identified and discussed.