The linewidth of distributed feedback semiconductor lasers with partially reflecting facets is analyzed theoretically to study its dependence on the grating phases at the facets. For most phase combinations, the linewidth is found to be reduced compared to its value expected in the absence of grating, i.e., the value expected for a similar Fabry–Perot laser. The extent of reduction increases with the coupling coefficient &kgr;. The calculated results are in qualitative agreement with the recent observations on phase tunable distributed feedback lasers.