Diode phase shifters are analytically and numerically studied in order to reduce to reduce DC power requirement, and to improve their performances. The residual series resistance of PIN diodes fabricated by the state-of-the-art silicon technology, is analized for the phase shifter use in terms of the punch-through voltage, the junction capacitance, and the forward bias current. Proper diode design for low DC power requirement is found to design the punch-through voltage of 10V which decreases the necessary forward bias current down to 10 mA per diode. The diode is applied to the examination on loss performance of the loaded line and the hybrid coupled phase shifter, where some analysis is made to indicate what diode is suitable for the phase shifter, and what relation there is between the diode impedance and the insertion loss of phase shifters. According to the analysis, and X-band MIC 4-bit 360phase shifter is constructed with an alumina ceramic substrate metallized with Au-Cr through photoresist techniques. This phase shifter requires only 0.81 W max. DC power, but it has the insertion loss of only 2.0 dB in the frequency range between 9.09.5 GHz. In the band, the VSWR performance is less than 1.5, and the phase errors are within5. It is shown that the reduced DC power phase shifter has the equivalent performances as well as the conventional one, except for the handling power capability.