A wide band passive fundamental wave mixer based on the novel broadband balun is proposed in this paper, the new design was simulated and validated by the measurement. Measured results show that about 9dB typical conversion, loss high gain compression and about 10dB return loss are achieved across the bandwidth from 3GHz to 50GHz, and LO-to-RF isolations are higher than 20dB.The results of conversion loss and isolation characteristic are the best performances reported among the same type mixer operating in ultra wideband millimeter wave frequency band.