The surface morphology of thin films has been investigated by means of scanning tunneling microscopy (STM) and X-ray diffraction (XRD). The full width at half maximum of the XRD rocking curve, corrected for substrate and instrumental aberration contributions using a simplified method, is found to correlate well with the fractal dimension which characterizes the surface roughness of the film. In addition. from the rocking curve data we determine the minimum grain sizes parallel to the film and compare these with those obtained from the auto-correlation function analysis of the STM topographies. Results for Au films grown on mica at different temperatures demonstrate the feasibility of quantitative correlation between STM and XRD rocking-curve data in metallic thin films.