Abstract: The microstructure and tensile fracture behavior of M390 powder metallurgy tool steels were investigated by scanning electron microscopy, X-ray diffraction analysis, and in-situ tensile test, and the effect of the carbides on the fracture process of the powder metallurgy tool steels was analyzed. The results show that, the microstructures of the M390 powder metallurgy tool steels are composed of the ferrite matrix and carbides as M₇C₃ and M₂₃C₆ with the chromium-rich alloy elements. These carbide phases seriously affect the fracture processes of M390 powder metallurgy tool steels. With the increase of the applied load, the cracks are firstly presented at the carbide phases distributed on the matrix, then propagate through or bypass the carbides. With the further increase of the applied load, the stress concentration at the tip of cracks causes the brittle fracture of the matrix. The carbide phases become the weakest in the fracture process of the M390 powder metallurgy tool steels. The fracture of the tool steel samples shows the brittle fracture characteristics and contains a small amount of dimples with the carbides inside.