Thermal stability of iron nitrides prepared by mixing laser and plasma beam nitriding (LPN) technology was studied. The treated samples were annealed in vacuum at different temperature from 473 K to 1273 K. The phases were detected by X-ray diffraction (XRD), the nitride's contents were calculated from the patterns of XRD, and the microstructures were analyzed by scanning electron microscope (SEM). Three critical temperatures (473 K, 673 K, and 1273 K) are found. Due to deeper nitriding layer in the LPN sample, the nitrides is more stable than that in laser-produced sample at the annealing temperature higher than 973 K. It is important and central for some potential industrial productions and applications. (C) 2007 Published by Elsevier B.V.