Experimental results and theoretical consideration of the slant path rain attenuation characteristics and site diversity effects are described, together with the prediction method. The slant path rain attenuation has been measured by using 17 and 27 GHz sun-tracking radiometers at Yokosuka Electrical Communication Laboratory in Japan for 3 years. At a distance of 33 km from the laboratory, the atmospheric noise was also measured by using 18 GHz passive radiometer for the above-mentioned period to know the site diversity operation characteristics. It is shown that calculated values of slant path rain attenuation and site diversity effects agree well with measured values. For example, calculated and measured values of the rain attenuation, at 17 GHz for 0.1% of time within the time interval when the elevation angle of the sun is from 30 to 60, are 5.5 and 5.1 dB, respectively. Experimental results using 18 GHz passive radiometers fixed at 20 and 55 elevation angles indicate that the rain attenuation has the elevation angle dependence.