Scattering from a concave structure, such as an open-ended cavity, which is electrically long and has an abruptly varying curved surface or an inner scattering object has been analyzed by the hybrid method integrating the HF and LF methods. In this paper the interfacing procedures of the SBR-FDTD method and the iterative PO-FDTD method were presented and the two methods to determine the iteration number of IPO simulation were proposed and verified by the comparison with that of mode analysis.