This paper examines how wage bargaining within each firm influences the relationship between an equilibrium ownership structure and the most preferred ownership structure from the viewpoint of social welfare, in a unionized oligopoly of asymmetric firms with respect to productivity of capital. We consider the merger incentive of each firm’s owner when the wage level is determined through bargaining between the firm’s owner and union. We derive a condition for both the degree of cost asymmetry among existing firms and the relative bargaining power of each firm’s owner to her/his union such that each ownership structure can be observed in equilibrium. We also show that although the two types of ownership structures with the merger involving the least efficient firm can be equilibria and socially optimal, these structures are observed only when both the degree of cost asymmetry and the relative bargaining power of each firm’s owner are moderate. Finally, we analyse the relationship among the cooperative game approach employed in this paper and two non-cooperative merger formation approaches, and examine the robustness of the results obtained in this paper against the change in the assumption regarding each firm’s cost function.