A structure of bidirectional filter to achieve single longitudinal mode for fiber laser gyro (FLG) application is demonstrated using a 0.1-nm bandwidth fiber Bragg grating (FBG) as a filter element. In filter section,the two direction lasers are kept orthogonal polarized as to keep counter-propagating mode coupling in minimum. Then a polarization beam splitter(PBS)is used to guide bidirectional laser out of filter. The insertion loss is about 3 dB and back reflection loss is more than 35 dB by test. The 1550 nm wavelength is gotten with the filter accessing to the ring cavity. Single longitudinal mode is certified for bidirectional laser output by using scanning ring resonator,which proves that the device is fit for bidirectional filtering.