Gravity-driven two-layer flow down a slightly wavy periodic incline at low Reynolds numbers is studied by a perturbation approach. Amplitude ratios and phase shifts of the fluid-fluid interface and the fluid-air free surface relative to the wavy wall are obtained as functions of the incline angle, wavelength of the roughness, and the ratios of film mean thickness, density and viscosity. Effects of surface tension and interface are included, and wall-shear-stress distributions are also discussed.