A method for controlling a rate of a voice service in a mobile communication system supporting the voice service via a packet network. The method includes the steps of receiving a control message at a terminal from a radio network controller (RNC); if the control message indicates control of a downlink rate, determining a downlink rate according to the control message; setting a Change Mode Request (CMR) field of an uplink Voice over Internet Protocol (VoIP) packet according to the downlink rate, and transmitting the uplink VoIP packet from the terminal to the RNC; if the received control message indicates control of an uplink rate, determining an uplink rate according to the control message; and generating an uplink VoIP packet including uplink voice data generated according to the determined uplink rate and frame type (FT) information indicating the determined uplink rate, and transmitting the uplink VoIP packet from the terminal to the RNC.