由於網際網路的快速發展下，訊息可以快速又大量的在網路上流通，使得搜尋引擎不但成為現今全球消費者蒐集產品資訊的主要方式，也成為企業進行廣告行銷的重要管道。本文將討論搜尋引擎所具有的跨市場網路外部性對消費者與廠商市場的影響。
本文將搜尋引擎設定為獨佔及雙佔兩種情況來討論，它提供廠商刊登關鍵字廣告及消費者搜尋資訊的服務，在搜尋引擎為獨佔時廠商要決定是否向搜尋引擎購買關鍵字廣告，消費者則是決定是否要使用搜尋引擎來蒐集訊息；搜尋引擎為雙佔時廠商的決策是該向那一家搜尋引擎購買關鍵字廣告，消費者要選擇在那一個搜尋引擎蒐集訊息。而消費者使用搜尋引擎的人數會影響到廠商購買關鍵字廣告的決策，同樣的廠商刊登關鍵字廣告的家數也會影響消費者登入搜尋引擎的決策。結果發現搜尋引擎為獨佔時，市場的廠商家數、消費者人數或廣告成交效率增加，都會提高搜尋引擎的利潤；搜尋引擎為雙佔時，在市場的廠商家數、消費者人數或廣告成交效率提升，則會引起激烈的價格競爭，使搜尋引擎的利潤皆下降。

This paper proposes a model to analyze network service by search engine in their attempts to deal with markets which are nevertheless interdependent through the network effect. Our model considers the search engine provides two types of network service, the “advertising service”used by firms to insert their websites in the search pages, the“search service”used by consumers to refer to products information in the search pages.
An increase in the number of firms, consumers or ads turnover rate leads to improve monopolistic search engine’s profit. The effects of changes in the exogenous parameter on the doupoly equilibrium price and profit are negative. With fewer firms, consumers or ads turnover rate, the search engines more competitive, and profits of search engines will fall.

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