Abstract: This paper describes the circuit design of a wide-band, wide-dynamic-range square-law diode detector, and analyses the operation theory of the circuit. The theoretic calculation and simulation show that when input RF source impedance is about 14 percent of the diode＇s dynamic resistance, the detector＇s square-law dynamic range is the widest. This paper also gives the design method of RF loop, video frequency loop and wide-band impedance transform network. The measured dynamic range is more than 29dB between 26.5GHz and 40GHz.