Aiming at the defect of traditional risk priority number(RPN) method in process failure modes and effects analysis(PFMEA),the reasons of the defect thoroughly were analyzed, and a new RPN ranking method based on cost and probability was presented which is combined qualitative analysis with quantitative analysis. In the qualitative analysis , which is based on the estimation of severity ranking in the traditional risk priority number method, the severity ranking of the process failure mode was estimated. In the quantitative analysis, it used the expected cost caused by the failure mode as the risk priority number. The improved RPN method uses the quantitative numbers to make the ranking result of the RPN more objective and accurate. Finally, the new RPN method was applied to the PFMEA of the filter assemblage of a module in the modern radar, and a great result is obtained.