Oxyfuel combined cycle with a gas turbine is a strong concept of CCS in gas-fired power plants. This technology is similar to a conventional combined cycle with a topping Brayton cycle, and a bottoming Rankine cycle, however oxygen instead of air is used in the gas turbine combustion. In the oxyfuel combined cycle, the gas turbine flue gases consist mainly of CO2 and H2O that supply energy to the heat recovery steam generator (HRSG), which produces steam for the bottoming cycle. After the HRSG, the water present in the flue gases is separated through condensation; then, most of the CO2 is recirculated to the gas turbine compressor, while the remaining CO2 is processed for storage. One of the problems to implement this technology is the necessity of the use of an air separation unit (ASU) to separate the oxygen from the air, which increases the energy consumption of the power plant. Thus, a comparative thermodynamic analysis was performed between a base case, composed by a conventional combined cycle, and an oxyfuel combined cycle to identify the pros and cons of each technology, the influence of oxygen purity in the oxyfuel combine cycle, and the main irreversibilities of each case. The cases were modeled and simulated in Engineering Equation Solver (EES). It was noticed that the oxyfuel combined cycle first law efficiency is 8.44% lower than the base case and second law efficiency is 7.33% lower than the base case. Simulations of the oxyfuel combined cycle model showed that an oxygen purity (molar basis) of 94 to 98% offers a good trade-off between energy consumption and quantity of CO2 captured.