A technique for preparing single heat mirror films with high quality is described. We also present an improved method for evaluating its optical and thermal radiative properties in the infrared region (2.5 < lambda, < 25 mu m). The calculated results agree well with that obtained by experimental measurements. The results calculated by using the simple Hagen-Rubens relation are also given. It is shown that the Hagen-Rubens relation is not accurate to predict the thermal radiative properties for the transparent semiconducting oxide films, such as indium tin oxides, prepared in our studies.