Petri net is a useful tool for system description in many research areas. PRMnet is a specified Petri net for the performance evaluation and prediction of parallel programs.The commonly used method for prediction with PRMnet is simulation. In this paper we first introduce the PRMnet briefly and then make some modifications.We introduce a method by calculation with the modified PRMnet. There are three phases in this method:first generate a transition list with Pnet;and then generate an execution graph with resource mapping; the length of the longest path of the graph is the total execution time, which can be calculated by some algorithms efficiently. Thus, with different resource mappings, we just need to redo the last two steps, which is much easier than simulation.