By using monochromatic sources of radiation we measured the reflectivity and transmission vs temperature behavior of YBaCuO thin film in the Far Infrared region in the range 300-20 K. A smooth change of optical properties is observed from critical temperature down to 40 K with an evident reduction of the losses for all the frequencies we examined. To analyze the use of YBaCuO films as semireflecting elements in submillimeter devices we made a preliminary study of the optical properties of the substrate in the same conditions. The permanence of the sample in a superconducting phase can be deduced from its room-temperature transmittance, as it changes drastically if some degeneracy occurs in the film.