本文利用一Hotelling模型，探討在競爭廠商具有商譽差異時，網路外部性產品的市場競爭問題。本文得到的結論如下。第一，在廠商雙占且完全涵蓋巿場的情況下，任一產品的網路外部性程度增加，會使得均衡價格降低，但任一廠商商譽提高會使得均衡價格提高。第二，若市場原為某一廠商獨占，則該廠商商譽提高卻可能使得對手廠商進入市場形成雙占。第三，即使產品具有網路外部性，社會最適的市場結構仍有發生雙占的可能。第四，從社會福利的角度來看，當產品的網路外部性大時，社會最適的情況應為產品中網路外部性程度較大的廠商獨占市場：當兩產品之網路外部性程度皆很小時，此時社會最適的市場結構為雙占。最後，我們發現當兩產品網路外部性程度均大時，則市場競爭下為獨占的情況仍可能為市場失靈。這是因為兩廠商網路外部性程度均大時，網路外部性程度較小的廠商仍可藉由擴充產量發揮網路外部性，進而獨占市場，造成市場失靈。

This paper uses a Hotelling model to explore the relationship between a firm's goodwill  and product network externality. We have the following findings. First, A product's price will decrease with its network externality when the duopoly market is covered. Second, if the market is monopolized initially by a firm, this firm's goodwill improving may instead induce its rival to enter the market. Third, the duopoly market structure may still be the social optimum even if the product has network externality. Fourth, the socially optimal market structure is a duopoly if two product externalities are both small. Last, we also find that monopoly may not be socially optimal when the two products' exter­nalities are both large.