A millimeter wave differential phase shifter working at 30~32 GHz is proposed in this paper. Its size is 30 mm×18 mm×0.127 mm. The phase shifter is designed on the basis of a microstrip line and consists of a central circular ring and a pair of split resonant rings (SRR). The optimization of S-parameter in the operational frequency band is realized by changing the radius of the central ring. Taking the transmission phase of the reference line as the standard, the differential phase shifts of 22.5°, 45°, and 90° are sequentially realized by changing the radius of the SRRs. The return loss of this phase shifter is less than -10 dB, the insertion loss is better than 1.4 dB, and the simulated maximum phase deviation is less than 5° at the designed frequency range. Moreover, the proposed structure is simple and convenient to fabricate. The reliability of the simulation result is verified by the sample test.