We investigated surface plasma resonance (SPR) properties of gold (Au) nanoparticles formed by step-by-step thermal annealing of Au films deposited onto substrates. We found that the averaged sizes of the Au particles formed in the annealing process depend on the initial thicknesses of the films. Using the geometric parameters extracted from the images of Au particles, the optical transmission spectra measured can be completely described in terms of SPR in the particles. The results suggest that annealing of Au films can be a simple and effective approach for producing Au nanoparticles with desired optical transmission properties, and the approach can be easily integrated into thin film fabrication processes.