An apparatus for forming a beam in a base station is provided and includes a plurality of channel cards for processing and outputting signals to be transmitted to each channel; a signal synthesizer/distributor for synthesizing the signals from the channel cards and compensating phases of the signals; a channel controller for controlling beams of the signals from the signal synthesizer/distributor according to a demand of a mobile communication terminal and outputting the controlled beam signals; a middle frequency generating block for receiving the signals from the channel controller and synthesizing the signals in each frequency to generate middle frequency signals; a transmitter for converting the middle frequency signals received from the middle frequency generating block into signals in a transmitting band; an RFB for amplifying the signals from the transmitter into signals in an output band and controlling phases of transmitting and receiving signals; and an antenna connection block for switching the signals to corresponding antennas of the RFB so that beams can be generated.