In order to release the increasing pressure of traffic flow in metropolises, this paper introduced a method of using high sensitivity microwave radiometric systems to monitor the traffic flow in all-weather conditions. In this method, radiometers were employed to detect and recognize the automobiles on the roads based on different behaviors of the automobile roofs and road surface in the millimeter microwave bands. The traffic control can be flexible according to the quantity of moving automobiles detected by the radiometers. Radiometers of 8 mm wavelength were constructed and used for detecting the traffic flow. This proposed method was proven to be low-cost, high accurate and fully capable in all-weather conditions in contrast with other competitors.