Abstract: In the design of unbonded partially prestressed concrete (UPPC) beams, it is necessary to estimate the deflection of the member under service loads so as to satisfy the requirements of a serviceability limit state. Because of the unbonding between the prestressing tendons with their surrounding concrete and the variation of the neutral axis with the applied moment, the centroid location of the cracked section varies and therefore the corresponding cracked moment of inertia varies too. It is not easy to determine the deflection of a UPPC beam. The moment of inertia of cracked sections is first determined for UPPC beams. Then, based on the Branson’s method, the effective moment of inertias as well as deflections for UPPC members can be easily calculated. The computed effective moment of inertia is close to that from Chinese Code for Design of Concrete Structures (GB50010-2010), and the ratio of the former to the latter is between 0.89~1.10. The computed deflections are compared with three researcher’s test results, it is indicated that the developed method is correct and also applicable to UPPC beams with unbonded FRP tendons. While the current Chinese Code is not applicable to the deflections for UPPC beams with unbonded FRP tendons.