本文建立一混合寡占模型，探討若產品具有網路外部性、且產品的相容程度不同時，兩廠商在市場上進行Cournot數量競爭，其最適產量、與政府的最適民營化政策將如何變化。本文發現，當兩產品完全不相容時，網路外部性效果會使得公營廠商增產、民營廠商產量變化不確定、兩廠商的產量差距加大；此情況下，最適民營化政策為部份民營，若廠商的邊際成本遞增速度較快，則最適民營化程度隨著網路外部性的增加，呈現先增加後減少的情況。而當兩產品完全相容時，若邊際成本的遞增速度較快，則公營廠商減產、民營廠商相應增產，而兩廠商產量差距減小，此與產品不具網路外部性的傳統民營化文獻之結果相異；此時，若網路外部性較大，則最適民營化政策為完全公營，此為本文與傳統民營化文獻相異處；此外，隨著邊際成本遞增的速度越小，網路外部性對相容下民營化程度的影響，其最適民營化程度之軌跡逐漸右移，呈現持續遞減、先增後減、至持續遞增的趨勢。再者，若成本遞增速度較快，不相容的最適民營化程度，會大於相容下的最適民營化程度；反之，若成本遞增速度較慢，則不相容的最適民營化程度，會小於相容的最適民營化程度。最後，與產品不具網路外部性的最適民營化程度相比，當產品不相容時，網路外部性會使得最適民營化程度下降，即不具網路外部性的最適民營化程度，會高於具網路外部性的最適民營化程度；當產品相容時，網路外部性會使得最適民營化程度下降或增加，即不具網路外部性與具網路外部性的最適民營化程度之大小關係不一定。

This paper studies the optimal privatization policy of the government given products have the characteristics of network externality. The products may be compatible or incompatible. We constructs a mixed oligopoly Cournot competition model, in which a public firm competes with a private firm, to examine the optimal privatization policy of the government and the optimal outputs of two firms. The main findings are as follows. Firstly, when the products of two firms are incompatible, the public firm produces more, the private firm produces less, and the differentiation of the products of two firms increases due to the network externality. Besides, the optimal degree of the privatization is partial privatization. Secondly, when the products of two firms are compatible and the marginal cost increases quickly, the public firm produces less, the private firm produces more, and the differentiation of the products of two firms shrinks. What’s more, the optimal privatization policy is to maintain the public firm. Least but not last, if the marginal cost grows quickly, the optimal degree of privatization is higher when the products of two firms are incompatible. On the other hand, if the marginal cost grows slowly, the optimal degree of privatization is lower when the products of two firms are incompatible. In sum, if the products are incompatible, the network externality will make the optimal degree of privatization smaller. If the products are compatible, the optimal degree of privatization will be either larger or smaller due to the network externality.

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