Abstract: Current mode circuit is a circuit which is used to process the current signal. Compared with the voltage mode, it has excellent performance in speed, bandwidth and dynamic range, so it has been developed rapidly in recent years. Translinear loop (TLL) theory is a rapid and effective means for analyzing current mode circuits. This paper first introduces the basic principle of translinear circuits. Then the bipolar transistor translinear loop principle is extended to MOS transistors. At last, the typical applications of translinear loop in current mode circuits are given.