A novel 2.4/5.2 GHz dual-band bandpass filter is proposed for WLAN (IEEE-802.11a/b/g) applications. The filter is cascaded by three open-loop square-ring resonators and two modified input/output transmission lines. The open-loop squarering with novel insert coupling structure is used to replace the typical hairpin structure for minimization. The filter is fabricated on the Al2O3 substrate with high quality factor by the printing method. The substrate is 0.6 mm in thickness and has a relative dielectric constant of 9.4. By combining the high dielectric constant substrate and modified structure, the proposed filter shows the advantages of low insertion loss, high out-of-band rejection, wide bandwidth and tiny size. The measured frequency responses reveal excellent agreement with the simulated ones.