A cell search method for handover between an asynchronous mobile communication system's base station and a synchronous mobile communication system's base station. A border base station, which is an asynchronous base station neighboring a synchronous mobile communication network, acquires information on a GPS satellite time, a reference time of the synchronous mobile communication network, generates base station frame number (BFN) synchronized to the satellite time and a system frame number (SFN) having a predetermined offset for the BFN for each cell, generates a system information block including a difference value between the BFN and the SFN, and transmits the system information block over a common channel along with the SFN. A dual-mode mobile station receives an SFN for each cell and a system information block from the border base station, acquires the satellite time synchronization, extracts a neighbor list of synchronous base stations, including PN offset information, from the system information block, searches a neighbor cell of a synchronous system at times when the PN offsets have elapsed from the satellite time synchronization, and reports the search result to the border base station. The cell search method contributes to rapid and efficient cell search during inter-system handover.