A novel dual-band planar microstrip filter using parallel coupled microstrip lines and open-loop stepped-impedance resonators（SIRs）loaded with two shunt open stubs is presented.By tuning the physical lengths of open-loop SIRs,parallel coupled microstrip lines and two stubs,the bandpass filter has good dual-passband performance at 2.55 and 5.35 GHz and high isolation between the two passbands.The relative bandwidths of the two passbands are 11.8% and 16.8%,respectively.Compared with the conventional open-loop SIR filters,the designed filter has a comparatively broader fractional bandwidth at the second passband.So it can cover all the wireless LAN（local area network）bands.In addition,the filter has the features of low loss,high rejection and low ripple.The measured results are in good agreement with the simulated responses by HFSS software.
