Thermally induced defects in Si crystals annealed at low temperatures around 500 °C have been investigated by photoluminescence measurements. It has been found that photoluminescence spectra of annealed Si crystals strongly depend on the concentrations of oxygen and carbon impurities. Many sharp luminescence lines, whose spectral energy positions and temperature behaviors are quite different from those of the usual bound‐exciton luminescence, have been found for the first time. Furthermore, strong, broad luminescence bands have been observed for annealed CZ‐Si with high oxygen and low carbon contents.