Abstract： In order to study the effects of the mounting angle of stern flap on the resistance performance of stepped planing hull, we carried out a series of model tests for a doublestepped planing hull with flap mounting angle β=2°, 3°, 4.5°. A finite volume method (FVM) based computational fluid dynamics (CFD) software CFX was used to simulate the flow field around the tested models at the volume based Froude numbers ranging from 0.44 to 5.20. The operating mechanism of the flap and the effects of mounting angle on the hydrodynamic characteristics were analyzed. Comparisons of air cavities after steps and load coefficient of planing surfaces between different models were presented and discussed. The experimental and numerical results showed that increasing the mounting angle could improve the lowspeed resistance performance and increase the critical velocity at which porpoising would happen, whereas only less resistance increase could be observed at high speed.