Process failure mode effect and critically analysis (PFMECA) is one of the most popular methods to study the reliability of process system. The method of Risk Priority Number (RPN) is the key part of PFMECA. Many experts have studied it in a long period of time. But the method has some drawbacks, such like sensitivity to small changes. The paper analyzes the defects of traditional Risk Priority Number method in PFMECA. And it proposes a new method based on cost and probability. The improved RPN method makes the result of the RPN analysis more objective and accurate. The new RPN method is applied to the PFMECA of a circuit board that is very important. Good result has been achieved.