We have observed an unusual hysteresis loop in rf power dependence of high‐temperature superconductor (HTS) coplanar transmission lines. At 77 K and 1 GHz, the signal transmission was linear (insertion loss <0.1 dB) until the input power (Pin) reached a critical value ofPc=0.4 W. An abrupt drop by ∼37 dB in the transmitted power (Pout) was observed atPin=Pcfollowed by a slow increase (Pout∝Pin0.62) forPin≳Pc. AsPmwas then decreased, it retraced the previous curve only untilPin=Pc. However, asPinwas reduced furtherPoutdid not rise untilPin=158 mW, at whichPoutstarted to increase rapidly and returned to the linear regime forPin<71 mW. Such a sharp transition and a strong hysteresis effect were unique to the HTS line whereas a gold line with an identical structure did not exhibit these characteristics. This behavior may be utilized for potential applications of new HTS microwave power (current) limiters and switches. Several possible origins for the hysteresis effect are proposed and discussed. ©1995 American Institute of Physics.