By a linear Cournot model, the factors associated with the integration of two firms are discussed in terms of their effects on the private surplus and the social welfare in economies with varying extent of openness. Meanwhile, the trade policy of the host government is taken into account. It is concluded that the more open the economy and the more intense the competition, the less profitable the integration; the most important determinant to the changes of the domestic social welfare is the industry strength of the host country. The host authority can enhance the domestic social welfare in two ways: to adjust the trade policy and to only approve the socially desirable cross-border mergers and acquisitions.