A new technique is developed to measure the mean junction temperature during ac operation of alternating current light-emitting diodes (ac LEDs). In general, the ac electrical system supplies power at a fixed root-mean-square voltage (Vrms) but the current may vary during operation. The ac LED begins to function after the instantaneous voltage rises above a certain threshold. Both the theoretical and experimental results show that the threshold current under the threshold voltage input increases linearly with temperature. The mean temperature of the ac LED increases from the ambient temperature to a quasi-steady state under ac operation. The mean junction temperature at the quasi-steady state can be determined by using the linear temperature dependence of the threshold current and measuring the threshold currents at both the initial and quasi-steady states. After the junction temperature of an ac LED is determined, the thermal resistance from the junction to the lead frame can also be calculated by definition of the thermal resistance. The thermal resistance of an ac LED under ac operation, calculated by the present method, is very close to that measured under dc operation by the conventional forward voltage method.