A new kind thermal radiation source consisting of photonic crystals (PCs) containing negative-index materials (NIM) coated atop SiC substrate is investigated theoretically by a transfer matrix method.The photonic crystals is inserted a defect layer and the wavelength range of the gap of it is tuned to approximately match with the wavelength range of the phonon absorption band of polar material SiC.Hence in all the incident lights that could pass through the photonic crystals,only the light the wavelength of which is equal to the gap defect mode can be absorbed by the SiC.So based on Kirchhoffs law.the structure can only radiate the light with same wavelength as the gap defect mode.Because of the novel characteristics of the gap defect mode,this kind thermal radiation source is predicated to have promising radiative properties.After a series of calculation,it can be seen that the structure shows very sharp emissivity peak in a narrow wavelength band and at a well-defined direction.The quality factor of the emissivity peak is great bigger than that of thermal radiation sources consisted of ordinary materials.This kind emissivity peak is insensitive to the emission angle,polarization and scaling factor of the photonic crystals in the structure.More importantly,the proposed structure promises the wavelength-selective thermal emission for both TE and TM polarization states along a well-defined direction.The value,quality factor and position of the emissivity peak can been effectively controlled by tuning the thickness and refraction index of the defect layer.