It is possible to determine thermal impedance value of a LED using a formula and some voltages measured on a specific measuring circuit. The formula has been obtained in pulsed high current operation of LED. Measuring circuits consist of an astable multivibrator which can drive a LED with pulsed high currents with appropriate pulse duration/period ratio and a photo detector circuit which converts LED's radiant power to proportional voltage. Three measured voltages have been used in formula to calculate thermal impedance. One of them is photo detector voltage which is proportional to LED's radiant power and the second one is a voltage drop on a resistor which is serial to LED and which is proportional to LED's current. Third measured voltage is voltage drop on the LED. The error ratio of calculated thermal impedance has been decreased in pulsed high currents. In this new method only voltages have been measured. There is no need thermal and optical measurements as available methods.