Parity-time (PT) symmetry has been demonstrated in the frame of classic optics and its applications in laser science have resulted in interesting mode selection mechanisms. PT symmetry breaking gratings (also called unidirectional Bragg gratings) have strong reflection when light is injected from one end, but they are transparent when injected from the other end. Such gratings are obtained through periodic modulations of both real and imaginary part of refractive index with a fixed phase relation. In this paper, we investigate distributed feedback (DFB) laser structures based on PT symmetric diffractive gratings. Our modal gain analysis shows that the PT DFB laser structure exhibits much stronger modal discrimination than the one using conventional diffractive gratings, so it will provide better single-mode operation performance. In addition, we analyze the influence of uncertain facet reflectivities on these DFB structures. We show that PT symmetric DFB structures are more stable against the perturbation of facet reflections.