When various kinds of outputs are produced from a single energy system, the methodology which allocates the common cost to each output cost is very important because it is directly related with the profit and loss of producers and purchasers. In the cost allocation methodology of the heat and the electricity on a cogeneration, there are energy method, work method, proportional method, benefit distribution method, exergetic methods, and so on. On the other hand, we have proposed a worth method which can be applied to any system. The definition of this methodology is that the unit cost of a product is proportion to the worth. Where, worth is a certain evaluating basis that can equalize the worth of products. In this study, we applied worth method to a steam-turbine cogeneration which produces 22.2 MW of electricity and 44.4 Gcal/h of heat, and then we allocated 2,578 $/h of common cost to electricity cost and heat cost. Also, we compared with various cost allocation methods. As the result, we conclude that exergy of various kinds of worth basis evaluates the worth of heat and electricity most reasonably on this system.