本文擴充 Motta（1993，Journal of Industrial Economics）的模型，建 立一個結合垂直產品差異性及網路外部性的模型，並藉以比較在Bertrand 價格競爭與Cournot數量競爭下，廠商的最適品質選擇、市場均衡及社會 福利的異同。當網路外部性夠大時，我們得到Motta（1993）一文所無法 談及的結果：(1)高品質廠商的利潤在Bertrand均衡下將大於Cournot均衡下 的結果；(2)低品質廠商的數量在Bertrand均衡下小於Cournot均衡下的結 果；(3)生產者剩餘在Bertrand均衡下都將大於Cournot均衡下的結果。

This paper compares Bertrand and Cournot equilibria in a vertically differentiated duopoly with network externalizes and attempts to provide more inference than what obtained by Motta （1993, Journal of Industrial Economics）. In addition to the results of Motta （1993）, if the network externality is large enough, we show that （i）the profit of high-quality firm and producer surplus under Bertrand competition are higher than that under Cournot competition; （ii）the equilibrium quantity of low-quality firm under Bertrand competition is lower than that under Cournot competition.