The effects of surface roughness on the optical properties of CdS thin film was investigated using RMS roughness measurements. CdS thin films for a window layer of the CdTe thin film solar cell were grown on ITO glass by a chemical bath deposition method with varying thiourea (CS(NH(2))(2)) mole concentration. The surface roughness of the CdS thin film was measured by Atomic Force Microscopy (AFM). After obtaining the correlation factor between the RMS roughness and the difference in the diffused transmittance and the spectral transmittance by using the integrating sphere, we investigated the effect of surface morphology on the transmittance in all CdS thin films. It is obtained that the higher RMS value the specimen has, the more difference between the diffused transmittance and the spectral transmittance is measured. Moreover, the difference is evident at the long wavelength region.