The dispersion of guided waves has been probed to estimate the thickness of a thin gold layer deposited on a two inches silicon wafer. Ultrasonic waves were generated and detected in a non-destructive and non-contact way using a laser ultrasonic setup. The guided waves dispersion curves were experimentally obtained with a two-dimensional Fourier transform, revealing the propagation of several modes. The development and the study of an analytical model allowed the determination of the most sensitive modes to the layer thickness. Using these modes the gold layer thickness was determined with no exact prior knowledge of the substrate and layer elastic characteristics. This method was found to be fast and easy to implement, with a suitable measurement error.