This paper describes numerical analyses of resistive mixer operation, followed by measured performances of a V-band (50 - 75 GHz) monolithic InP HEMT resistive mixer operable with a very low LO power. Our model assumes that the channel conductance of the InP HEMT can be described by three linear functions according to the applied gate voltage. The calculated results obtained with the model have shown that the LO power level required for mixer operation is determined by the gate bias voltage and that a device with abrupt conductance shifts is suited to low LO power operation for a resistive mixer. It is also shown that conversion loss saturation of a resistive mixer is caused by its channel conductance saturation. A V-band monolithic resistive mixer has been designed and fabricated using Coplanar Waveguides (CPW) and a 0.15 mm InP HEMT with abrupt channel shifts. Good agreement between measured and simulated conversion losses are obtained. A minimum conversion loss of 8.4 dB is achieved at the 55 GHz RF-frequency and the -2 dBm LO power. It also exhibits an excellent IF output linearity to allow the 1 dB compression RF input level to be comparable with LO power, indicating good intermodulation performance. It is demonstrated that the proposed simple model of the channel conductance can easily calculate conversion characteristics of a resistive mixer with high accuracy.