Standard reducer bends have structural functions of both bend and reducer pipe at the same time.However,there is a lack of strength design method in the current codes.In order to develop the strength design method of the reducer bends subjected to internal pressure,analytical formulas are re-examined. Finite element analysis and stress measurements of the reducer bend are carried out.It is found that it is not appropriate to analyze the reducer bends by using thin membrane theory. The formula derived directly from circumferential stress formula of reducer bends under internal pressure is conservative,which is further verified by the finite element analysis results and it can thus be applied to piping design. Standard reducer bends have structural functions of both bend and reducer pipe at the same time. However, there is a lack of strength design method in the current codes. In order to develop the strength of the design method of the reducer bends subject to internal pressure, analytical formulas are re-examined. Finite element analysis and stress measurements of the reducer bend are carried out. It is found that it is not appropriate to analyze the reducer bends by using thin membrane theory. The formula derived directly from circumferential stress formula of reducer bends under internal pressure is conservative, which is further verified by the finite element analysis results and it can be applied to piping design.