Abstract: The microstructure and mechanical properties of HP40Nb steel furnace tube in a hydrogen reformer after 15-year service were studied by optical microscope, scanning electron microscope, energy dispersive spectrometer, tensile and impact testers. Based on the high-temperature creep rupture testing results, the remnant life of the furnace tube was evaluated. The results show that after the long-term high-temperature service, in the microstructure of furnace tube steel, the grain boundaries coarsened with the G phase precipitation; secondary carbides precipitated in grains obviously; creep voids were observed near inner and outer wall and in matrix; oxidation and chromium depletion were observed near inner and outer wall; decarburization layer was formed on the outer wall. Compared with the as-cast steel furnace tube, the tensile strength of the furnace tube after the long-term high-temperature service reduced by 30% and 18% at room and high temperature, respectively, the impact energy at room temperature decreased significantly, and the remnant life was less than 1 a.