The purpose of this paper is to find out whether the financial holding companies in Taiwan have operating synergies occurred as a result of a corporate combination, and what the main sources of the synergies are. Using input and output data from fourteen financial holding companies and their subsidiaries of bank, insurance and bills finance during the period 1998-2004, a fixed translog cost function is employed to analyze cost efficiency of the companies before and after mergers. The empirical results show: (1) most companies have exhibited operating synergies that come mainly from economies of scale; (2) all subsidiaries of bank and insurance with scale economies are beneficial to horizontal (in-market) mergers, and the combinations of bank and insurance through cross-market mergers also have the effects of scope economies; (3) all subsidiaries of bills finance with diseconomies of scale are not feasible for in-market mergers, and the combinations of bills finance and bank or bills finance and property-liability insurance through cross-market mergers perform the effects of scope diseconomies.