1. A method for setting an initial frequency by a user equipment (UE) in a mobile communication system in which the UE performs an initial cell search in order to identify a Node B with which the UE can exchange data, the method comprising: performing a first step (step# 1 ) cell search of a 3-step cell search for identifying the Node B on a plurality of predetermined frequency hypotheses through a primary synchronization channel; storing the cell search results; determining an initial frequency offset estimation value from the cell search results for the predetermined frequency hypotheses; and estimating a frequency offset using the determined initial frequency offset estimation value, wherein the predetermined frequency hypotheses are pulse duration modulation (PDM) hypotheses located at regular intervals in a loop bandwidth of an automatic frequency controller, and wherein the PDM hypotheses are PDM values.