According to the principle of fracture mechanics, the fracture toughness of the furnace tube after 30000 hours service and new furnace tube were tested with the MTS810 tester. The microstructure of fracture surface were analyzed by the SEM. The results show that the fracture toughness of the furnace tube after service gets decreased greatly compared with that of the new furnace tube. The microstructure of fracture surface of the furnace tube changes significantly corresponding to the change of the fracture toughness.