A 2-D Finite element simulation method was developed based on the kinetic law and the energy evolution during the whole process of deformation, which is used to investigate the creep size effects in polycrystalline thin metal film on substrates. Three diffusion paths (e.g. surface, grain boundary and lattice diffusion) are considered in the present model. The diffusion rate for these three processes was compared under different loading conditions with corresponding microstructure. It’s found that grain boundary diffusion is coupled with another diffusion channel. Creep size effects result from mass transferring in thin film. The model gave the quantitative results of the influences of the film thickness, grain size, and the constraints of the substrate on polycrystalline metal film diffusion. The simulated results present the evolution of the point defects in grain interior, the strain and stress field. The distribution of the crack-like stress in the grain boundary could explain the stress concentration mechanisms clearly and this also agrees with the literature results.