We　report　a　construction　of　distributed-feedback　(DFB)　optical　microcavities,　which　is　realized　through　mechanical　contact　between　a　high-quality　planar　thin　film　of　a　polymeric　semiconductor　and　a　large-area　homogeneous　nanograting.　Using　poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]　thiadiazol-4,8-diyl)]　(F8BT)　as　the　active　medium　for　the　planar　layer,　we　achieve　strong　amplified　spontaneous　emission　from　such　a　microcavity　with　a　low　threshold.　This　not　only　simplifies　largely　the　fabrication　techniques　for　DFB　microcavities,　but　also　avoids　the　unexpected　chemical　interactions　during　solution　processing　between　the　organic　semiconductors　and　the　nanograting　materials.　Furthermore,　high-quality　polymer　thin　films　with　high　surface　smoothness　and　high　thickness　homogeneity　are　employed　without　any　modulations　for　constructing　the　microcavities.　This　also　suggests　new　designs　of　microcavity　light-emitting　diodes,　or　even　for　realizing　electrically　pumped　polymer　lasers,　simply　by　metallizing　the　dielectric　nanogratings　as　the　electrodes.