Resonant filters can be beneficial for significant reduction of the filter size in buck converters. Compared to the conventional low-pass filter, the resonant filters can be designed to provide high attenuation to the ripple components near the pulse width modulation (PWM) switching frequency resulting in much smaller filter components. In traditional resonant filter, the high attenuation at a specific frequency cannot be maintained due to the variation of filter components under changing operating conditions. In this paper, a tunable capacitor based resonant filter is introduced for design of the buck converter. The capacitor value can be actively controlled to tune the filter operations near the frequency of interest. Through experiments, compared with the traditional low-pass filter, performances of the proposed tuned resonant filter are demonstrated to achieve equal filtering performance at a much reduced filter size.