A mathematical model is developed for a 200 MW coal-fired, drum-type natural circulation boiler with reheat. The complete unit is divided, for the convenience of the analysis, into several sections, viz. the economizer, the drum, the downcomer, etc. The dynamic behaviour of each section is described in mathematical terms. The equations are linearized and manipulated into state space form for solution in a digital computer. Results for the more important system variables are presented end discussed. Difficulties found in the modelling are emphasized.