This paper describes the prediction of temperature distribution and flow characteristics in a full-scale utility boiler at design condition. The boiler under investigation is a 120MW boiler of tangential type where natural gas is the main source of fuel. It was originally designed to use oil as the main fuel source but modified in 1995 to accommodate burners for natural gas firing. The prediction is made by means of Computational Fluid Dynamics simulations. The simulation is done in three stages with increasing complexity namely mixing, combustion and combustion with the effect of heat transfer to water wall. Generally, the prediction of temperature distribution and flow pattern inside the furnace agrees well with practical observation. Combustion gas flow inside the furnace shows a highly swirling flow pattern in anti-clockwise direction. Rapid increase in temperature is shown at the wind box region where combustion process takes place. Temperature distribution at furnace’s cross-section is highly influenced by the rate of air flow into the furnace. The temperature varies from 1600K at bottom furnace to 2100K at the furnace exit.