Novel polymer distributed feedback (DFB) gratings are fabricated based on photopolymerization to reduce lasing threshold of polymer lasers. A photopolymer formulation sensitive to 355-nm ultraviolet (UV) light is proposed for the fabrication of polymer gratings and it can be used to form polymer films by spin-coating process. A very low surface-relief depth ranging from 12.5 to about 1.0 nm has been demonstrated with a refractive-index modulation of about 0.012. The experimental results indicate that such polymer gratings have promising potentials for the fabrication of low-order DFB organic semiconductor lasers.