The high-temperature behavior of boronized layer of 45# carbon steel has been studied in detail by using high-temperature X-ray diffractometer (HTXRD), difference temperature analysis (DTA) technique, high-temperature microscope, high-temperature hardness tester. Research results show that (1) the hardness of the boride layer decrease with temperature, (2) there is no oxidation of boride layer on the measuring range of the temperature, and (3) the threshold temperature of the phase transformation of the boride layer from FeB to Fe2B is about 860 degreesC.