A method for performing radio bearer mapping in an user equipment of a wireless communications system includes setting a plurality of multiplexing options of a frequency division duplex radio bearer, called FDD RB hereinafter, when the user equipment operates in a Cell Dedicated Channel state, each of the plurality of multiplexing options corresponding to a multiplexing mode, determining whether an RB of the user equipment has one of the plurality of multiplexing options, and using a multiplexing mode corresponding to a multiplexing option of the plurality of multiplexing options for performing RB mapping when the RB has the multiplexing option.