The investigation was aimed at evaluating the effect of selective oxidation and sulphur/phosphorus-containing compounds on coking behaviour at the surface of HP40 alloy during thermal cracking of light naphtha. The surface morphology and composition of the selectively oxidized alloy were characterized by SEM, EDX, and XRD. Coking tests show that the coking rate of blank alloy decreased with sulphur/phosphorus-containing compounds, while the effect of sulphur/phosphorus-containing compounds on coking rate for selectively oxidized alloy is complex, and a mass concentration of mu g/g results in the lowest coking rate. The morphology and microstructure of cokes were investigated by SEM and Raman spectra. SEM and Raman spectra analyses indicated that either selective oxidation or sulphur/phosphorus-containing compounds resulted in more disordered graphite structures and amorphous carbon.