Some results of our study on single frequency, narrow linewidth fiber grating external cavity semiconductor laser are reported in this paper. The effects of external cavity feedback on the threshold gain and emission linewidth are discussed theoretically; Laser emission with side mode suppression ratio more than 25 dB, spectrum linewidth narrower than 60 kHz is realized at 1.55 μm wavelength region experimentally by coupling a conventional multimode semiconductor laser with a fiber Bragg reflective (FBR) filter fabricated by ourselves.