The influence of adding sulfur/phosphorus-containing inhibitor on coke formation was evaluated at 850 oC during light naphtha cracking after the SiO2/S coating was prepared on the surface of HP40 cracking tube after 8 years service. The results were compared with that of adding sulfur/phosphorus-containing inhibitor only. The oxide layer on the internal surface of HP40 alloy after 8 years service is mainly composed of (Fe, Ni, Cr) spinels and the needle-like intermetallic compound of Cr and Fe intermetallic. Compared with the blank run, the amount of coke was decreased by about 69% for continous adding 200 ppm sulfur/phosphorus-containing inhibitor only. However, by approximate 65% for the SiO2/S coating only, and by about 96% for the SiO2/S coating + 50 ppm sulfur/posphorus-containing inhibitor. the surface of HP40 specimen served 8 years is covered by the filamentous coke for the blank run, and the coke is formed around microcracks with adding 200ppm sulfur/phosphorus -containing inhibitor only. However, a small amount of granular coke is formed on HP40 specimen with adding 50 ppm sulfur/phosphorus-containing inhibitor and the SiO2/S coating. The application of the SiO2/S coating + 50 ppm sulfur/phosphorus-containing inhibitor can potentially extend the run length of radiant coil in the industrial cracking furnace.