A LLC resonant converter (LLC-RC) has been widely used in DC/DC converters for the benefits of high efficiency and high power density. The efficiency of a LLC resonant converter is greatly dependent on the switching frequency. In order to achieve higher efficiency, the switching frequency should be equal to the resonant frequency. This paper presents a novel resonant frequency tracking technique for LLC resonant converters (LLC-RC) based on the detection of zero current duration on the secondary side of the converter. The LLC-RC is controlled by the proposed technique to achieve resonant frequency tracking automatically under the various operating condition. To verify the control technique, a 5 kW LLC-RC was designed and tested through a Matlab/Simulink environment.