Industrial boilers of a particular type will behave differently due to manufacturing or assembly tolerances. In addition, the performance of a boiler will vary at different times of its service life. The objective of the present study is to simulate a boiler unit. In the boiler model, heat transfer in the combustion chamber is simulated by the zone method; heat transfer in the secondary superheater, the reheater, the primary superheater, and the economizer is simulated by lump parameter analysis. One feature of the method used is that the models of the major components have been coupled sequentially according to the boiler configuration. Thus, it is assumed that this method may be applied to different boiler systems with different configurations.The uncertainty factors, such as water tube deposits, component deterioration and so on, are considered by modification factors which are determined from on-line measurements. Another feature of the boiler system simulation is that the major parts of the boiler system are simulated and coupled together to analyze some important operational parameters which impose a significant effect on boiler efficiency, such as main stream temperature, reheat temperature and reheat pressure drop. The boiler model has been tested and compared to on line measurements data. The results are within a reasonable error range.