Based on the traditional quarter wavelength step impedance resonator (SIR), a new dualband bandpass filter (BPF) with wide bandwidth is proposed. Its center frequencies are 1.90/5.65 GHz, which can be used in wireless communication systems such as GSM. The filter has a wide frequency range, with the absolute bandwidth of the first and second center frequencies being 980 and 960 MHz respectively. At  1.90 GHz, the proposed BPF has return loss less than -25 dB and insertionloss less than -0.012 dB. The BPF has better isolation between the two passband and a simple structure for easy manufacturing. Simulated frequency responses S₁₁ and S₂₁ of the filter agree with the measured results.