A method and apparatus in a UE for detecting active channelization codes including channelization codes used by other UEs, which are different from at least one channelization code assigned to the UE, for transmission/reception to/from a Node B in an assigned time slot in a mobile communication system, where each of time slots of a radio channel frame is identified by a plurality of code channels, each having a unique channelization code. Accordingly, a channelization code set is formed to include a predetermined number of channelization codes to be searched, code channel data corresponding to the channelization codes is detected from a received signal, the code power of the code channel data is measured and compared with a predetermined threshold, and channelization codes having code power values exceeding the threshold are selected as active channelization codes.