Tunnel diodes are some of 2-port devices with negative differential resistance (NDR). In this paper, we propose a low insertion loss isolator, which can be designed to operate up to sub-millimeter region, by using resonant tunneling diodes (RTDs) and a HEMT. Small-signal analyses are performed to confirm insertion loss and unidirectional characteristics for the proposed active isolator. It is found that unidirectional amplifications as well as isolation characteristics could be expected below sub-millimeter waveband as a result of numerical calculations.