Abstract: 4Cr5MoSiV1 hot work die steel is generally subjected to the cyclic thermal stress caused by molten aluminum and coolant, which results in thermal fatigue failure. The controlled cyclic gas nitriding treatment was adopted for the tempered and thermal-fatigue samples to prepare nitriding layers with and without Fe-N compounds respectively.The thermal fatigue performance test was conducted. The results of thermal fatigue tests indicate that the nitriding compound-free layer exhibits better resistance to thermal fatigue crack generation than the nitriding compound layer.The secondary nitriding before crack production can complement the nitrogen atoms, reset the compression stress and increase the hardness, which is beneficial for the promotion of thermal fatigue resistance.