The basis for control optimization is a nonlinear modeling for a drum-type boiler consisting of furnace, economizer, evaporator, superheaters, attemperator, turbine bypass and thick-walled components such as boiler drum and superheater header. After the mathematical formulation of the boiler start-up optimization problem and of the cost function, including the related hard constraints for inputs and states, the optimal reference and input control are calculated using a multi-stage control vector parameterization method and SQP techniques. The results of this optimization process are optimal reference values and the corresponding input trajectories. The control structures applied minimize both the fuel consumption and start-up time of the boiler. (C) 2004 Published by Elsevier Ltd.