Nitrogen-doped 6H-SiC single crystals irradiated with neutrons up to a fluence of 5.74×1018 n/cm2 at the temperature of 60-80°C were investigated by means of X-ray diffractometer and metallurgical microscope. The experimental results showed the X-ray diffraction peak (0006) was broadened due to the lattice distortion resulting from irradiation-induced defects, and then narrowed linearly when isochronally annealed over the temperature of 700°C. Meanwhile, from the chemical etching photomicrographs, the characteristic was accompanied by the changes of the dislocation density after the process of irradiation and post-irradiation annealing. According to this characteristic of irradiated 6H-SiC crystals, a novel temperature sensor suitable for the temperature range of 700-1300°C or more is developed, which depends on the linear relationship between XRD FWHM (the full width at the half maximum of X-ray diffraction peak) and isochronal annealing temperature over about 700°C. The subsequent application test demonstrated that the sensor remained no damage in the very harsh conditions as well as possessed a less than +5% of the relative temperature error. Therefore, the neutron-irradiated 6H-SiC can be employed as a kind of non-invasive temperature measurement sensor to determine the temperature of closed, high-speed rotating and difficult-to-access parts on a running machine such as internal-combustion engine pistons, turbine blades and so on.