Abstract: To study the effect of various HP40 alloy surface status on the catalytic coke formation, HP40 alloy samples were oxidized by steam and air respectively at different temperatures, followed by coking tests. The oxidized surfaces of the samples and the morphology of coke were characterized by SEM, EDS and XRD, furthermore, the C/H ratio and the structure of coke were analyzed by elemental analysis and Raman spectroscopy. Test results showed that when alloy sample was oxidized under steam, a layer of needle type Fe rich oxide was formed on the surface; oxidation under air, Cr rich oxide particles were well dispersed on the alloy surface, with the increase of oxidation temperature, spalling of oxide layer from the alloy surface might happen, in this case, the exposed porous surface could be an idea area for forming catalytic coke. The coke formed was mainly catalytic filamentous coke with a diameter of about 80 nm, minor thermal cracking coke might exist too. The coke having a very condensed structure consisted of both ordered and amorphous types, and with low graphitization degree. However, the atmosphere of pre-oxidation had more influence on the oxidized surface of alloy, yet just little on the followed coking process, coke morphology and structure.