X-ray reflectivity (XR) measurements with a synchrotron radiation source were carried out for thin polymer films on a glass plate. From the XR data, the film thickness and surface and interface roughnesses could be determined. In addition, the appropriate conditions and precision for measurements were also discussed. Kiessig fringes were observed clearly for specular XR measurements of poly(methylmethacrylate) thin film. Analysis of the XR data allowed the determination of the film thickness very precisely. By a curve-fitting procedure of the XR profile, the film-surface roughness and film–substrate interface roughnesses were determined. A Fourier transform of the XR data was performed as an alternative method of evaluating the film thickness. The values for the film thickness obtained by the curve-fitting procedure and Fourier-transform procedure were slightly different from each other. One possibility for the cause of this difference may be an integral error and/or cut-off effect in the Fourier-transform procedure. The XR technique with synchrotron radiation is a very powerful tool for structural characterization of thin polymer films.