For the optimization of combustion in utility coal-fired boilers, a simple analytic model was set up to relate the radiant energy signal (RES) with the combustion rate (heat release rate) based on the heat transfer equation inside a boiler furnace. It was pointed out that as the air flow rate into the furnace changes, the highest RES corresponds to the highest efficiency, making RES a sensitive quantity for optimization of combustion in boilers. Experiments carried out in a 660 MW down- and coal-fired utility boiler confirmed the characteristics of RES as an indicator of combustion rate inside the furnace and its ability to reflect the boiler thermal efficiency varied with the air flow rate. The utilization of RES in the optimization of combustion can generally improve the boiler thermal efficiency at different unit loads, and the efficiency was raised about 1.0% especially at the rated and lower unit loads. It should be stated that except the lower unit load, the NOx emission from the boiler after optimization of combustion by RES would increase due to the limitation in supply of adequate air flow rate into the boiler, and some new combustion technologies are now available to solve the contradiction.