An apparatus and method for determining access power in a mobile communication system. The system includes a base transceiver station (BTS) for generating and transmitting a message field including access parameter information and BTS load information within an area covered by the BTS, and the mobile terminal for determining transmission power for access on a basis of power of a signal received from the BTS and the message field. The method includes the steps of generating a message field including access parameter information and BTS load information and transmitting the message field from the BTS to the mobile terminal within an area covered by the BTS, and determining transmission power for access in the mobile terminal on a basis of power of a signal received from the BTS and the message field.