Oxyfuel combustion is considered to be one of the most effective methods to improve thermal efficiency for high temperature processes such as boiler water of steam power plant. In the oxyfuel combustion, the combustion characteristics such as temperature and CO2 emission are largely influenced by the process parameters such as the mixing between fuel gas and oxygen and the nitrogen content in the oxidant and fuel. A numerical simulation of 200 MW gaseous tangential firing boiler with varying O2 content 25%, 30%, 35% is presented. Simulations demonstrate the characteristics of temperature distribution and CO2 form rate in the furnace. The results show that by increasing O2 percentage in air combustion there is an increase in temperature level and CO2 forms rate by 6.8% and 13.92% in Oxy25, 16.9% and 15.02% in Oxy30, and 19.14% and 15.72% in Oxy35 compared to airfuel combustion.