A measuring instrument of the frequency and current dependences of the modulus and phase of the thermal impedance of LEDs and LED matrices of 100 W and higher is described. The principle of operation of the measuring instrument is based on the transmission through LEDs of a sequence of impulses of direct current that is pulse-width modulated under the harmonic law, and the determination of the variable component of the temperature of the p–n junction of the LED as the voltage changes during intervals between current impulses. An algorithm is presented to determine the component of full thermal resistance of an LED or an LED matrix from the results of processing the frequency dependences of the modulus and phase of the thermal impedance. Examples of implementation of the algorithm are given.