The　local　thermal　conductivity　of　polycrystalline　aluminum　nitride　(AlN)　ceramics　is　measured　and　imaged　by　using　a　scanning　thermal　microscope　(SThM)　and　complementary　scanning　electron　microscope　(SEM)　based　techniques　at　room　temperature.　The　quantitative　thermal　conductivity　for　the　AlN　sample　is　gained　by　using　a　SThM　with　a　spatial　resolution　of　sub-micrometer　scale　through　using　the　3　omega.　method.　A　thermal　conductivity　of　308　W/m.K　within　grains　corresponding　to　that　of　high-purity　single　crystal　AlN　is　obtained.　The　slight　differences　in　thermal　conduction　between　the　adjacent　grains　are　found　to　result　from　crystallographic　misorientations,　as　demonstrated　in　the　electron　backscattered　diffraction.　A　much　lower　thermal　conductivity　at　the　grain　boundary　is　due　to　impurities　and　defects　enriched　in　these　sites,　as　indicated　by　energy　dispersive　X-ray　spectroscopy.