Abstract: The microstructure and fracture morphology of K438 alloy were studied by metallographic microscope and scanning electron microscope, and the reasons for the low room temperature plasticity were analyzed.The results show that the precipitated phase of K438 alloy is mainly composed of granular, massive and long strip MC carbides.The presence of long strip MC carbide in K438 alloy is the main reason for the decrease of elongation at room temperature.