In this paper, by combining the equality differential increment theory (EDIT) with the equivalent polyhedron body (EPB) searching extremum strategy, a method of economical load dispatch for the steam turbine unit of the thermal power plant has been presented. Simultaneously, a method for the order of load shedding and the related optimal economic operational modes of each turbine unit in decreasing its load are ascertained. According to the methods, the results of the optimal economical dispatch of the peak loads, the order of load shedding and the related optimal economic operational modes of each turbine unit while decreasing the load of a certain thermal power plant have been obtained. These results are applied in the thermal unit, and its economic benefits are shown to be obviously higher than that of applying the relative efficiency of each turbine unit. The methods can be used as a reference for obtaining the optimum economical load dispatch for similar thermal power turbine units. Copyright (C) 2007 John Wiley & Sons, Ltd.