Abstract: This paper derives by means of scattering matrix the theoretical limits of convertion loss and noise figure of idealized microwave resistive mixer with its image frequency, sum frequency and the other harmonics under different terminating load conditions (that is, all of chem are resistively termined, reaotioely terminated, or some of them are resistively terminated, reaotively terminated and other combinations). Attempts have heen made to illustrate some basic pyhsical concepts involved under partial and total energy recovery and non-recovery working conditions.The results are summarized in table 1. It is found that under image matched conditions it is possible to have a single-sideband noise figure of less than 3dB, and that the ultimate noise figure is 3.92 dB with all ports matched.