Abstract: Metallographic examination and tensile test were carried out on the long-term service radiation section tube of ethylene cracking furnace, and the microstructure, carbide size, room temperature and high temperature mechanical properties of carburized and non-carburized zone were studied. The quantitative relationship between carbide width and tensile properties of furnace tube was established. The results show that the grain morphology and size of non-carburized zone and carburized zone of the furnace tube were similar. The former was composed of austenite matrix, primary carbides, G phase and NbC phase; the latter had no G phase, and its primary carbides were obviously coarsened. The carburized zone had more creep holes and microcracks. The room temperature strength and plasticity of the furnace tube were seriously deteriorated. The high temperature tensile properties of the non-carburized zone basically met the requirements, and the carburized zone was obviously embrittled. The carbide width increased with the distance from outer surface of furnace tube. The tensile strength and yield strength of the furnace tube were positively related to the carbide width, and the percentage elongation after fracture was negatively related to it.