Reliability is one of the most important indicators for software quality. Among the present researches of software reliability, majority focus on the appliance of probability statistics model for the whole software system. Few work based on software model for analyzing the software reliability is learned. Reliability Petri Net (RPN) is presented in this paper. In RPN, transaction means the function or module of software, and is marked with reliability gene. Based on analyzing the Petri net structure, four reliability operators are developed to perform the relationships between tractions. Reliability formulas are provided respectively for the reduction and decomposition operations of Petri net. Furthermore, priority of these reliability operators is given. With this research, more complex Petri net model could be greatly simplified and the reliability of the system could be evaluated effectively and easily. An example is provided for demonstrating the practicability of this reliability analysis method.