The effect of dilution gas addition is an important topic in the field of gaseous fuel combustion. In the present study, the effects of CO2 addition and oxygen enrichment on the characteristics of laminar coflow methane and ethylene diffusion flames were investigated experimentally. The shape and temperature distribution of the flames under different atmospheres were analyzed. With the increase of oxygen concentration from 21% to 50%(j), the height decreased by about 70% for both flames, while the flame temperature and brightness increase. Under the same oxygen concentration, the brightness of the ethylene flame is higher than that of the methane flame. With the increase of CO2 concentration from 0 to 20%(j), the flame height increased by 28%, and the average edge temperature of the flame at each height decreased by 290℃, and the centerline temperature decreased by 132℃. The flame brightness is reduced from yellow color to dull yellow, and the visible flame base sligts down towards the burner rim. When the CO2 concentration is more than 20%(j), the flame will be lifted, and eventually blown out.