Thin films of nucleic acid bases have been grown on cleaved KBr (100) by evaporation deposition. The structures of the thin films are investigated by infrared spectrometry and X-ray diffraction. The orientation of these molecules on the substrate has also been determined by the measurement of FT-IR (fourier transform infrared reflection) absorbance. It is revealed that adenine and thymine molecules are evaporated on the substrate to form thin films without decomposition.