A single-longitudinal-mode dual-wavelength fiber laser used to generate millimeter waves was presented. Polarization-maintaining fiber Bragg grating (PM-FBG) was used as wavelength selected component in this laser. Two wavelengths of PM-FBG is orthogonal polarization states, enhance the polarization hole burning effect and effectively reduce the different wavelength competition and achieve stable dual-wavelength output. The Vernier effect of the compound cavity effectively suppresses multi-modes oscillations and realizes single-longitudinal-mode operation. The separation of two wavelength is 0.318 nm (39.8 GHz) and side-mode suppression ratio (SMSR) is more than 50 dB.