CaCu3Ti4-xMnxO12　(x　≤　0.1)　ceramics　has　been　prepared　via　the　conventional　solid-state　method.　The　Mn　substitution　has　strong　effects　on　the　microstructures　and　dielectric　properties　of　CaCu3Ti4O12　ceramics.　The　grain　becomes　obviously　uniform　and　fine　after　a　very　small　amount　of　Mn　substitution　for　Ti.　The　intergranular　phase　gradually　disappeares.　The　bulk　resistivity　increases　from　107　to　109　Ω　cm.　Dielectric　permittivity　is　abruptly　suppressed　to　102,　and　dielectric　loss　is　significantly　reduced　to　10-3　for　x　=　0.004-0.04.　The　better　microwave　dielectric　properties　are　also　achieved　for　CaCu3Ti4-xMn　xO12　(x　=　0.004)　ceramics　as　follows:　εr　=　68.1,　Q　×　f　=　4030　GHz,　τf　=　220　ppm/K.　These　results　suggest　that　a　small　amount　of　Mn　substitutions　for　Ti　is　an　effective　way　to　tune　the　microstructure　and　dielectric　properties　of　CaCu3Ti　4O12　ceramics.　©　2010　Elsevier　B.V.　All　rights　reserved.