The thermal neutron energy spectrum in a homogeneous medium has been studied by many authors. In case of heterogeneous reactors, the absorption in fuels is so strong that the thermal neutron energy spectrum in the fuel will have a higher energy than in the moderator. The different temperatures of these media will also cause a difference of spectra. The thermal neutron energy spectra of each medium in a heterogeneous reactor are studied in this paper. The thermal neutron energy spectrum and the neutron temperature in light-water moderated and UO2 fueled reactors were calculated by a numerical method and the approximate Wigner-Wilkins method.