Silicon wafers become semitransparent at room temperature and at wavelengths more than 1.1 μm. Silicon wafers with an oxide film layer are also semi-transparent because the extinction coefficient of the film optical constants is negligible at visible and infrared wavelengths. We experimentally studied optical properties such as emissivity, reflectivity and transmissivity of silicon wafers with and without oxide films to devise new radiation thermometry that is applicable to semi-transparent silicon wafers near room temperature. The proposed radiation thermometry which is constituted from two blackbodies and p-polarized optical components showed the accuracy of ± 1 K at the temperature range from 313 K to 343 K using a radiometer with an InSb sensor sensitive at a wavelength of 4.7 ± 0.1 μm for silicon wafers with low resistivity. It turned out that radiation thermometry near room temperature for silicon wafers with resistivity over 1 Ωcm is very difficult because their emissivities are extremely small.