Abstract: Based on a low order nonlinear model of a boiler turbine unit, a simplified general model which can capture the essential dynamics and interaction of a unit is derived in this paper. The control structure of this model is analyzed via the decentralized relative gain method (dRG) under the condition of finite bandwidth control. Then independent design principle is applied to design the decentralized PID controllers and the stability of closed loop can be guaranteed. When the coupling is strong and the fully decentralized structure can not meet the desired performance, Gramian method is applied to analyze the partially decentralized structure of the model, and based on the decentralized and the centralized controller, the resulting controller is tuned. Simulation results show that partially decentralized controller can achieve comparable performance as the centralized one; furthermore it has a simpler structure and is easier to implement and tune.