Abstract: A dual passband frequency controllable microstrip filter based on a folded SIR resonator is proposed. The dual passband filter is realized by combining the SIR resonator characteristics and transmission line theory. The filter is designed with two degrees of freedom. Regulating the microstrip width of resonator allows to control the frequency and its interval of the two passbands. This paper also investigates the effect of regulating the microstrip length of resonator on the filter frequency characteristics. The measurement results show that the microstrip filter has two passbands with center frequencies of 2. 79 GHz and 3. 90 GHz, the minimum insertion loss of passband are -0. 96 dB and -3. 0 dB respectively, the minimum return loss of passband are -42 dB and -18 dB respectively, and the fraction bandwidths are 5. 7% and 6. 7% respectively. Agreements of simulation and measurement results prove the effectiveness of the proposed filter and the proposed design approach.