現今的科技產品環境，當新產品進入市場時，因為舊有產品使用者建構的網路外部性下，不容易開拓其市佔率。過去討論網路外部性的文獻中，大多僅探討一項新產品與舊有產品競爭的情況。然而，現實中往往出現多項產品同時在市場上競爭。本文模型建立在Jassen and Mendys (2007) 上，來探討擁有較高網路外部性之舊產品，面臨同時出現兩項品質較好之新產品時，市場佔有率的分佈樣貌。此模型中，存在於市場上的消費者對品質及網路外部性的偏好為異質，且模型中的消費者每期皆會重複購買該產品。本文發現，三項產品同時存在於市場上的穩定均衡解不存在，且市場均衡解將會出現在僅剩品質最優的新產品及既存舊產品共存的市場下，品質居中的產品無法存活於市場。新產品品質與舊產品品質間的差距不大時，舊產品仍然主導市場；若新產品品質明顯優於既有產品品質時，則新產品會拿下整個市場。且當品質優勢超過八分之一時，新產品將很快地占領整個市場。 / The market today is a harsh environment for new firms introducing new technologies into it, because users of existing products have already constructed a tough network which cannot be shaken easily. On network externality, previous researches only analyzed the condition of one new product competing with one old product. However, in reality, there are often more than two products competing with each other in the market. Based on Jassen and Mendys (2007), our model further discusses the outcome of the competition between two new superior products and an old inferior product. Consumers have different preferences of the quality and network externality of a product, and buy one product in each period. The outcome reveals that there is no stable equilibrium solution, when three products co-exist in the market. Stable equilibrium solution exists when there are only two products, the best one and the worst one, in the market. The product with middle quality cannot survive. Also, old products can still possess the most part of the market when the quality differs subtly between new and old products. But when the quality of new products is apparently higher than that of old products, then new products could take over the whole market. We discovered that the quality difference must be greater than 0.125 for new products to dominate the market quickly.