We report on the annealing effects of gold thin films deposited on silicon by analyzing the FTIR, AFM and d.c. conductivity, before and after annealing, FTIR spectra show higher IR transmission for samples after short time annealing, but the transmission decreases after longer annealing. The AFM photographs reveal that gold clusters are formed after short annealing and consumed by reaction with Si after longer annealing, The d.c. I-V measurement shows a nonlinear conductivity of gold thin films. The thin film resistance becomes lower after longer annealing. The I-V behavior can be explained by percolation of silicide clusters with fractal-like structures. We suggest a mechanism of silicide formation for that silicide is formed out of Au cluster and then diffused along the Si surface, which leads to less IR transmission but higher conductivity. The results of FTIR, AFM and I-V measurements are consistent with our model.