The IBAC (In-Band Adjacent-Channel) DAB (Digital Audio Broadcasting) system is to provide multichannel CD quality audio services and multimedia data services including text and picture in FM band (88~105 MHz). As the FM band is being used by the existing analog radio broadcasting, there must he an analysis of the interference effect between IBAC DAB and analog FM signal. Therefore, the protection ratio should be evaluated to verify the system compatibility and allocate the new IBAC DAB channel in FM band. In this paper, among the three types of interferences, FM-to-DAB, DAB-to-FM and DAB-to-DAB, that can be occurred, the Protection ratio of IBAC DAB system for co-channel FM interferer is analyzed by modeling the FM interferer and considering the multipath fading channel. The simulation results show that IBAC DAB system has far better sensitivity than Eureka 147 and needs a relatively high protection ratio for co-channel FM interferer, because of its narrow bandwidth, about one third of that of Eureka 147.