Abstract: To calculate the flexural strength of externally prestressed concrete beams, the ultimate stress in external tendons must be known firstly. The relationship between the ratios of the equivalent plastic hinge length to the neutral axis depth is established for prestressed concrete beams with external FRP tendons and steel tendons. Based on different investigators’ test data, the ratio of the equivalent plastic hinge length to the neutral axis depth is analyzed for prestressed concrete beams with external FRP tendons. It is found the ratio for prestressed concrete beams with external FRP tendons is stable and can still be treated as a constant，but this constant is not the same as that previously found for prestressed concrete beams with external steel tendons, and it can be transformed by the suggested relationship equation. Finally, a method is proposed for the determination of ultimate stress in external tendons. It is applicable to simply supported beams as well as continuous beams. It is applicable not only to externally prestressed concrete beams with conventional steel tendons, but also to those with non-conventional FRP tendons.