Non-equilibrium molecular dynamics approach is adopted to simulate the out-of-plane thermal conductivity of nanometer silicon film. The simulation results reveal that the out-of-plane thermal conductivity of nanometer silicon film is significantly lower than the bulk value and decreases with the decrement of film thickness. From the simulation results, the thermal conductivity variation tendency changes near the 20 nm. With the increase of film thickness, phonon mean free path is not a fixed value. Attributing the thermal conductivity size effect to be phonon-boundary scattering the theoretical analysis of the thermal conductivity has been performed using the Boltzmann transport equation and Holland model. The thermal conductivity theoretical results agree well with the results of molecular dynamics simulation, which indicates that the phonon mean free path in nanometer silicon films is significantly decreased. (c) 2006 Elsevier B.V. All rights reserved.