Tunable filters with varactor-loaded spilt-ring resonant structure have been fabricated and the electromagnetic bandgap tuning is achieved. The varactor is positioned in a gap of a single split-ring resonator or between two split-ring resonators, and the filtering is tuned through capacitances modulation of the structure by adjusting the bias voltage applied to the varactor. The frequency response function of the filter to electromagnetic wave is numerically analyzed, and the physical principle of electromagnetic bandgap formation is explained. In order to eliminate the magnetic resonant effect and close the gap of split-ring resonator, experiments are carried out. It is found that the some bandgaps are resulted from magnetic resonance and some are resulted from electric resonance.