The theoreticalmodel for the polarization-based tunable birefringence fiber filter is presented by using principal states of polarization. The tuning mechanism resulting from the polarization state has been theoretically discussed, and the characteristics of the tunable filter have been studied with thismodel. The results show that the tunable characteristics of filter are determined by phase difference between horizontal and vertical components of input signals. The polarizer and amp lification of the input signal vectors have no effects on the tunability . L inearly tuningwith the maximum transmittance can be realized by changing the phase difference if app rop riate polarizer and directional angle of the input signal vectors are selected.