To evaluate the possibility of CH4 exploitation in linkage with CO2 isolation, the decomposition rates of CO2, CH4 and CO2-CH4 mixed gas hydrates were obtained under isothermal isobaric conditions. The decomposition rates of CO2 and CH4 pure gas hydrates are proportional to the total amount of gas hydrate and the fugacity difference from the equilibrium state under the conditions of three-phase coexistence. The rate constant of CO2 hydrate decomposition is larger by one order than that of CH4 hydrate. In the CO2-CH4 mixed hydrate decomposition, CH4 composition in the generated gas is substantially larger than the value calculated from the pure hydrate decomposition rate while CO2 remains selectively in the mixed gas hydrate.