With the widespread penetration of distributed generation (DG) and the deepening reform of distribution market, DG operators emerge as new individual self-interested stakeholders, and it is of significance to study the market transaction between participants in this new market environment. This paper proposes a two-level transaction model between DISCO and DG owners, where the upper-level formulates the decision-making of DISCO and the lower-level describes the responses of DG owners. The upper-level problem is to minimize the operation cost of DISCO, whose decision variables involve the energy purchased from main grid, the transaction prices for DGs, and the curtailed load. The lower-level problem is to maximize the transaction profits of DG owners, whose decision variables are optimal outputs of DG units. Considering the coupling between two sub-problems, the bi-level model is transformed into a single-level mathematical program with equilibrium constraints by an equivalent primal-dual approach. Meanwhile, the power flow constraints are modeled as the second order cone form. Case studies show the Stackelberg game relationship, and can effectively guide the determination of transaction prices between DISCO and DG owners.