Abstract: Cracks were found in the regular inspection of Incoloy 800HT alloy high temperature steam pipeline used for 13 a, and the causes of cracks were analyzed by tensile test, bending test, metallographic examination and micromorphology observation. The results show that the tensile and bending specimens both fractured on the weld fusion line. A large number of precipitated phases existed on the fusion line, and the precipitated phases were mainly carbides formed by alloying elements chromium and molybdenum. The phase led to the decrease of high temperature mechanical properties of the alloy and the generation of stress concentration. Creep holes were generated near the precipitated phase and evolved into creep cracks after long-term high temperature use.