Far infrared photoresponse of magnetoresistivity ΔRxx of two-dimensional electron systems is investigated theoretically at high temperatures. Photon-assisted impurity and phonon scatterings, rather than the electron heating, are shown to be the dominant mechanisms for high-temperature photoresponse at cyclotron resonance. Based on the theory developed for electron magnetotransport driven by intense terahertz radiations, we have calculated the far infrared photoresistivity of a GaAs/AlGaAs heterostructure at temperature T=150 K, predicting notable cyclotron resonance of ΔRxx, in agreement with the experimental finding. Contrary to low temperature case, where the electron heating is almost solely responsible for the cyclotron resonance of ΔRxx, at T=150 K electron-heating contribute at most a small fraction of the total response. © 2003 American Institute of Physics.