Process failure mode effect and critically analysis (PFMECA) is a widely used method to study the reliability of the manufacturing system. The method of Risk Priority Number (RPN) is the key part of PFMECA. It is used to determine the risk degree of each process failure mode. However, the traditional RPN method of PFMECA has some drawbacks. The paper analyzes the drawbacks of the traditional method and proposes a new method based on AHP (Analysis Hierarchy Process) and cost. The AHP method is able to determine the importance of each process. It is effective for solving complicated problems. The cost is useful to evaluate the risk of a failure mode. The improved RPN method makes the result of the RPN analysis more objective and accurate. Furthermore, the paper gives a fictitious instance using the new RPN method on PFMECA of a circuit board. The desired result has been achieved. This new method provides an effective and convenient tool for failure analysis and improves PFMECA implementation in failure and risk analysis for manufacturing production lines.