Abstract: In delayed coking process, water vapor is injected into the heater as a dilution medium to suppress coke formation, yet sour water pollution is generated, now dry gas is proposed to use as an alternative dilution medium to eliminate the sour water pollution. In order to investigate the effect of dry gas on the yield and compositions of gas formed by thermolysis, thermal reaction experiments are conducted under the atmosphere of CH4 and N2 using vacuum residues from Jinling, Jinan and Qingdao as feeds, the gas yields and compositions are measured. Results indicate that under the same reaction conditions, compared with in N2 atmosphere the gas yield in CH4 atmosphere is lower and the volume fraction of methane in cracking gas is 1-2 percentage point lower. Furthermore, under same reaction time, the difference of gas yields between these two conditions decreases as the reaction temperature increasing, as well as decrease with the increase of oil vapor partial pressure.