A tunable photonic crystal filter was proposed with parallel nematic liquid crystal as defect layer. Optical performance and the eigenmodes were analyzed by 4×4 matrix method. The results show that there are two kinds of transmission modes when the voltage is low. Different modes can be selected by rotating the filter axially. Peaks of two modes merge into one and the polarization sensitive performance disappears as the applied voltage increases. Photonic bandgap is insensitive to voltage. Tunable range of the filter is 87 nm. The positions and number of transmission peaks depend on the thickness of defect layer sensitively.