Abstract: A novel digital microwave phase shifter is proposed for high resolution phase adjustment of single tone microwave signals. The device is constructed by concatenated DDS digital phase shifter and analog phase locked loops. The phase step resolution of this device is 2π×2^-24 radians, far beyond the capability of typical DDS phase shifters. System design schematic and operation principle is explained as well as experimental restdts. The idea can be implemented to precisely control the phase differences of two or more single tone carriers. High resolution phase adjustments of band limited signal can be realized by some necessary circuit expansion.