The flip-chip structure for millimeter-wave MMICs has been investigated to obtain high performance and high reliability. In our approach, an air gap between the MMIC and the alumina substrate was determined so as not to change electrical characteristics from those of the unflipped MMIC. We calculated the proximity effect between the MMIC and the substrate by using 3D-electromagnetic simulator, and found that the air gap should be controlled to be greater than 20 µm. Since the discontinuity of transmission lines at bump interconnects is not negligible above 60 GHz, we constructed the LCR-equivalent circuit for the bump interconnect and confirmed its validity by comparing measurement with calculation. Based on these investigations, the 60- and 76-GHz-band CPW three-stage low noise amplifiers were successfully mounted on the alumina substrate using a thermal compression bonding process. The gain of the flipped 60- and 76-GHz-band MMICs are greater than 18 dB at around 60 GHz and 17 dB at around 76 GHz, respectively. The noise figures are 3.6 dB and 3.9 dB, respectively. The gain and noise performances showed little degradation compared to those of the unflipped MMICs when appropriate bonding conditions are given. We confirmed that the flip-chip structure has high reliability under a thermal cycle test. From these results, flip-chip technology is promising for millimeter-wave applications.