This paper describes a monolithic microwave integrated circuit (MMIC) active module with small phase variation and low insertion loss for beamforming network in S-band. The MMIC active module composed of a digital phase shifter, a digital attenuator and a buffer amplifier, has characteristic to control amplitudes and phase shifts by using digital control signals. By using the digital attenuator, the MMIC active module has obtained the excellent performances. This paper also describes the exact on-state resistance of FET switch for designing the digital attenuator.