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The near-net-shape casting technology is an important process unattainable with traditional die-making processes. The key of this process is how to guarantee the safety application of dies. In this paper, the microstructures and thermal fatigue (TF) behavior of Cr-Ni-Mo hot work die steel modified by rare earth (RE) were investigated. The grains of cast Cr-Ni-Mo hot work die steel are refined by RE modification. With the increase of RE addition, both grain size and inclusion amount are reduced. The morphology and distribution of inclusions are improved by appropriate RE modification. RE modification favors fine plate martensite. When the residual RE content reaches 0.02 %, no obvious changes in strength and hardness are found, while fracture toughness is increased. The impact toughness, elongation and reduction of cross sectional area are increased by a factor of two. The influence of microstructure on initiation and propagation of thermal fatigue crack was also studied. The results show that the microstructures have apparent effect on resistance to TF crack initiation and propagation. Steel with microstructure of martensite and bainite dual phase have higher resistance to thermal fatigue than that of martensite owing to the loosing stress increase. Wrought process increases TF crack initiation resistance, however, lowers TF crack propagation resistance compare to cast process KEY WORDS: hot work die steel; thermal fatigue; microstructure; rare earth.