The degradation of X-ray image passing through thin glass substrate was experimentally investigated in term of modulation transfer function (MTF). The screen was prepared by sedimentation of the powder phosphor (Gd2O2S:Eu) on thin glass substrate with thickness 140 m (Waldemar Knittel- GmbH). The experiment was carried out under X-ray mammographic conditions. The beam quality was 28 kV, 60 mAs, with W/Rh target/filter combination. The screen was coupled to an optical readout device including a CMOS remote RadEye HR photodiode pixel array. Results showed that there is a 40% degradation of the MTF on average, in the whole spatial frequency range, due to the glass substrate. The glass substrate has a low impact on resolution of the image at low frequencies (12% on average, up to 2.7 cy/mm) and a high impact at medium and high frequencies (70% on average).