A novel optical polarizer based on anti-resonant reflecting optical waveguide (ARROW) structure at quasi-anti-resonant conditions is proposed. The great advantage over conventional ARROW devices is the fact that large polarization isolation of the proposed polarizer can be obtained even with a thick guiding core. This is accomplished by tuning the ARROW from the anti-resonant conditions toward the high-order anti-resonances of the second cladding layer. For the SiO 2/Si/SiO 2 ARROW polarizer with 8 μm thick core, polarization isolation ranged from 40 to 57 dB/cm can be achieved if the index detuning between −3.7% and −4.2% of the core with respect to the second cladding is assumed. The isolations between the TE 0 mode and the high-order TE modes in the transverse direction of the ARROW polarizer were also discussed.