Abstract: With more and more renewable generation integrated into power system, the number of thermal units decreased and their reserves are not enough, so it is necessary to utilize the reserve capacity from third-party entities. Two kinds of day-ahead power market clearing mechanism incorporating reserve ancillary service of third-party entities are proposed and compared in this paper. In the first clearing mechanism, energy and reserves provided by the thermal units are jointly clearing at first and then the reserves provided by third-party entities are cleared. In second clearing mechanism, joint market clearing mode for electric energy and reserve on both sides. At first, based on the aspects of market development stage and shortage of reserve, the advantages and disadvantages of 2 clearing modes are analyzed. Then the clearing models are presented and the impacts of third-party entity on bidding results in 2 modes is analyzed. Furthermore, the costs and benefits of different market members are analyzed. Finally, the numerical results of the modified IEEE 30-bus system are given to show that the reserves of third-party entities can be purchased fully to decrease total costs of generation and reserves under the second clearing mechanism.