We have studied thermal annealing effects on the surface morphology of 800 Å thick gold films, using scanning tunneling microscopy. The gold films were thermally evaporated onto glass substrates, and were then measured with the scanning tunneling microscope at room temperature before and after annealing. The annealing treatments were done at temperatures between 200 and 500°C and for times from 3 to 60 h. The topographic images were analyzed using various statistical methods and image-processing techniques. We present data showing the evolution of the average surface-grain size and rms roughness amplitude of the gold films as a function of annealing temperature and duration. The typical grain size was found to increase with time for all annealing temperatures with a rate that increases with temperature. The rms roughness amplitude, on the other hand, shows a more complex dependence on the annealing temperature. Our data suggest that surface diffusion is the main process active at low annealing temperatures, but other mechanisms are dominant in the high-temperature range.