A reduced slot cycle (RSC) mode control method is provided for observing a paging slot by a mobile station to receive a message transmitted from a base station in a mobile communication system. The method includes the steps of determining at least one parameter related to an operation time of the RSC mode and the number of operation periods where the mobile station operate with different slot cycle indexes (SCIs), through an exchange of at least one message with the base station, calculating a per-SCI operation time in the RSC mode operation time using the determined parameter, and starting the RSC mode, setting a reduced slot cycle index (RSCI) as an initial value of the SCI determined between the mobile station and the base station, wherein the RSCI has a value smaller than an SCI value of an idle state, increasing the SCI value step by step each time the SCI operation time of slot period arrives, and terminating the RSC mode if the increased SCI value reaches the SCI value of the idle state.