A method of handover in a wireless communication system in which an asynchronous system and a synchronous system are mixed is disclosed. The method comprises the steps of: the asynchronous system ordering a dual mode mobile terminal to perform a handover from the asynchronous system to the synchronous system; when the handover order is received at the mobile terminal, the mobile terminal transmitting a frame or a preamble through a reverse traffic channel to the synchronous system, reporting to the synchronous system that the handover is completed and initiating a timer of the mobile terminal which is set according to a predetermined time; the mobile terminal switching from an asynchronous vocoder into a synchronous vocoder when the timer expires; and the wireless communication system switching vocoders of asynchronous and synchronous MSCs when the report is received.