Generation side competition is the preliminary stage of the electricity market. Flexible loads participating in market competition are inevitable for electric power industry deregulation. Traditionally, the pricing mechanism of locational marginal price is adopted for day-ahead market clearing. Both theory and practice have proved that it is difficult for the locational marginal price mechanism to simultaneously achieve the requirements of individual rationality, incentive compatibility and maximization of social welfare. Because of the imbalance in information in the electricity market, and in order to overcome the strategic bidding of market participants and realize the high trade efficiency and social welfare maximization, an electricity pricing mechanism for considering generators and flexible loads participating in the day-ahead market clearing is proposed based on VCG mechanism design theory. It meets incentive compatibility, individual rationality, and social welfare maximization simultaneously. Finally, the basic characteristics of the proposed day-ahead market clearing pricing mechanism are demonstrated by the simulation results of a 2-bus system and the revised IEEE14-bus system. This work is supported by National Natural Science Foundation of China (No. U1965104).