In view of the load level， the price duration curve（PDC） is defined. On this basis， the definition of the average PDC under all loads in the study period is given. In accordance with the definition， a calculation method of PDC is proposed by taking account of unit forced outage and unit stochastic bidding. By substituting the equivalent units for the capacity block of generating units， this method makes it possible to obtain the probability of marginal price and PDC based on the power system probabilistic production simulation. This method is characterized by conceptual clarity， flexibility and time saving. Finally， an illustrative case study of PJM electricity market is presented.