This paper presents a simple structure for suppressing the second-harmonic of tunable filters. The proposed resonator for the filter possesses the advantage that its resonant frequencies of odd-mode can be electrically tuned, whereas the even-mode harmonics can be suppressed. In addition, it is very simple and compact in structure. Based on the proposed resonator, a tunable filter with harmonic suppression has been implemented and compared to the counterpart without harmonic suppression. The comparison shows the second harmonics can be effectively suppressed with little impact on the passband performance.