Heat resistant cast tubes have been used for ethylene cracking furnace. Creep strength, carburization resistance and anti-coking characteristics are required for ethylene cracking furnace tubes. We have developed a novel clad tube, named EST (Ethylene Super Tube), and have been increase the number of installation into the commercial furnaces. TMT (Tube Metal Temperature) and ΔP (Differential Pressure) in the practical application was compared with conventional ethylene cracking furnace tubes, followed EPMA analysis of inner surface of the tubes. Results of the practical application trials are concluded as follows. (1) EST in commercial furnaces showed lower TMT tangent and lower ΔP. The extension of run-length is estimated twice and the increase of ethylene yield is estimated for a few % by superior anti-catalytic coking. (2) PPW overlaid layer serviced for 6 years had Cr₂O₃ layer with no Fe. The oxide layer is considered to prevent the catalytic coke formation for a long term. (3) EST serviced for 6 years showed superior carburization resistance by high Cr content and longer tube life is expected.