THESIS ABSTRACT The bulk of Internet Service Providers adopt two-part tariff. Two-part tariff means users should pay a fixed rental every month, and pay a varied fee base on their connecting hours. Internet Service Provider group their customers by analyzing customer¡¦s demand level, and provides different group different Pay-Rating. We call this situation Multiple Pay-Rating. If Internet Service Provider provide their customers single Pay-Rating. We call this situation Single Pay-Rating. People always analyze two-part tariff in monopoly market. In this thesis I will analyze two-part tariff in oligopoly market.Then compare with the former, and we will know whether two-part tariff erode customer¡¦s welfare. I will create a differentiate-product demand function base on Logit model, and build two-part tariff oligopoly competition model. Analyze the equilibrium in Single Pay-rating and in Multiple Pay-rating. And I drew several conclusions from the analysis. 1. In oligopoly market structure two-part tariff not necessarily deprive of customer¡¦s welfare. On the contrary if ISP group their customers, customer¡¦s welfare will improve. 2. If there are no differentiation between customers¡¦ utilities, nonlinear pricing will degraded to linear pricing. If customers¡¦ utilities are different, ISP will set up the unit connect fee equal to the unit cost, and the month rental will raised with the increasing differentiation between customers¡¦ utilities. 3. If Internet service provider group their customer and provide them different Pay-rating, the final equilibrium will be symmetrical equilibrium or non-symmetrical equilibrium.