A miniaturization of Ka-band direct detection receiver based on monolithic microwave integrated circuit (MMIC) used for passive millimeter wave(MMW) imaging was proposed. This receiver was composed of wideband low noise amplifier (LNA) module, small signal diode detector and dc amplifier. Each module was designed, fabricated, and tested. The performance of the receiver was also tested. The measured results show that the receiver achieves a RF gain above 30dB with noise figure less than 3.8dB and an effective bandwidth 7.4GHz. Some MMW images obtained with this receiver using a horn antenna prove the receiver’s applicability in passive MMW imaging.