Abstract: Carburization kinetic curve of the two alloys (Cr25Ni35Nb and Cr35Ni45Nb) for ethylene cracking tubes was investigated and analyzed by the carburization experiments at elevated temperature (1 000 and 1 100 °C). Contents of Fe, Cr, Ni and Nb, growth of carburization layer, microstructure and composition of different zones of the two carburized materials were also studied by means of optical microscopy and scanning electron microscopy (SEM). The experimental results show that the carburization resistance of Cr35Ni45Nb alloy is better than that of Cr25Ni35Nb alloy; some locations on the surface of the specimens carburized during two cycling of time spall off and the spallation of Cr25Ni35Nb alloy is much more serious, this indicates that the thickness reduction of the ethylene cracking furnace tube will occur due to high temperature carburization and its strength and service life will be reduced. A mass of dispersive distributed secondary carbides generates in the zone of noncarburization to center zone of coupons, the density of which decreases with the distance increased from surface of coupon; at 1 100°C, the initial bone framework structural morphology of the two materials disappears after carburization and the secondary carbide along the grain boundaries spalls off after carburization.