Based on the data of an online peer-to-peer lending marketplace，Prosper． com，the users’inter and intra-group externalities and platform’s pricing strategies in the third-party electronic market are empirically investigated． The estimated results show that both the users’network effects and platform’s profits are influenced by the demand-supply relationships，the operation mode of the platform and the users’trading behavior in the market． There are positive inter-group network externalities between the lenders and borrowers，negative intra-group network externalities among the borrowers and positive intra-group network externalities among the lenders． Borrowers and lenders are both sensitive to the rates of transaction fees set by the platform，and the profit of the platform has quadratic relations with the rates of transaction fees． Because there is less supply than demand in the marketplace，the platform’s profit is mainly affected by the size of the lenders and the transaction fee rate of the lenders．