A novel configuration of doublebalanced sub-harmonic mixer using a ring connected anti-parallel diode pair (RAPDP) is presented for operating in 26-30 GHz. The monolithic microwave integrated circuit (MMIC) was implemented with GaAs 0.15 μm PHEMT technology with the compact size of 0.85×0.85 mm2. The proposed mixer consists of two spiral transformer baluns and a band-reject filter. The RF spiral balun with a band-reject filter served by an L-C resonator is used to provide an output port for the IF extraction. The best conversion loss of the sub-harmonic mixer is 12.7 dB and LO-to-RF and LO-to-IF isolations are better than 24 dB and 28 dB respectively.