By introducing the excess burden of taxation, we analyze endogenous choice of organizational form between U-form and M-form in the multiproduct mixed duopoly. With managerial delegation in public and private firms, we find that choosing M-form (U-form) for the public and private firms is a dominant strategy under mixed duopoly if the degree of excess burden of taxation is low (high). Furthermore, in the intermediate range of excess burden of taxation, there exist asymmetric organizational form (i.e., the public (private) firm chooses U-form (M-form)) or multiple organizational form. In equilibrium under mixed duopoly, consumer surplus with M-form is always greater than that with U-form, while social welfare crucially depends on whether the excess burden of taxation is larger than a certain threshold. Finally, regardless of the degree of the excess burden of taxation, social welfare and consumer surplus under mixed duopoly are greater than those under privatization.