The metallurgical properties and the microstructure of coke after gasification reaction with pure H2O and pure CO2 were investigated in this study. Moreover, the first-principles calculation was conducted to study the reaction process of the carbon with pure H2O and pure CO2. The results show that the CRI (coke reaction index) increases sharply and the CSR (coke strength after reaction) decreases sharply, when the cokes are gasified with H2O as compared to CO2. The scanning electronic microscopy images and the coke panoramagrams show that H2O more easily leads to the generation of large pores (>500 mu m) and destroys the coke structure than CO2. The X-ray diffraction results indicate that the arrangement of carbon atoms of coke becomes regular and the ordered degree of coke increases after reaction with CO2 and H2O; however, after being gasified with H2O, the cokes have a higher ordered degree than with CO2. The results of the first-principles calculation show that the H2O molecule is more likely to react with carbon as compared to the CO2 molecule due to the lower energy barriers of H2O adsorption and H-2 formation. The M-2 > FS reaction process is the controlled step of the C-H2O reaction process, as well as in the C-CO2 reaction system.