Long term stability is the main factor that influences the minimum detestable signal of microwave radiometers. Two new types of microwave radiometer are develoed: a computer gain Compensative microwarve radiometer and a continuously calibrated microwave radiometer. The long term stability of both designs is optimal because they are insensitive to system gain fluctuations. The continuously calibrated microwave radiometer is also insensitive to system noise fluctuations. The minimum detectable signals are 0. 13 and 0. 19K respectively under an integration time of 0. 6s.