According　to　the　debate　on　the　origin　of　the　giant　permittivity　of　CaCu3Ti4O12　(CCTO)　ceramics,　a　small　amount　of　MnO2　substitution　for　CuO　or　TiO2　was　adopted　to　prepare　the　ceramics　with　nominal　composition　of　CaCu3-xMnxTi4O12　(x=0-0.3)　or　CaCu3Ti4-yMnyO12　(y=0-0.1)　by　solid-state　reaction　method.　The　mechanism　of　giant　dielectric　response　in　CCTO　ceramics　was　discussed　in　view　of　microstructure　evolution　and　variation　of　electric　properties.　The　results　show　that　CCTO　ceramics　with　a　small　amount　of　MnO2　substitution　exhibit　BCC　perovskite　single　phase,　but　the　abnormal　growth　grain　gradually　disappears　and　grains　become　obviously　uniform　and　fine.　Meantime,　resistivity　significantly　increases　from　107　to　109　Ω·cm,　dielectric　constant　is　abruptly　suppressed　from　104　to　102,　and　dielectric　loss　is　greatly　reduced　from　10-1　to　10-3.　An　internal　barrier　layer　capacitor　(IBLC)　is　associated　with　relative　insulating　grains/subgrains　and　semiconducting　grain　boundaries/subgrain　boundaries.　CCTO　ceramics　with　high　dielectric　constant,　low　dielectric　loss　and　good　temperature　stability　are　achieved　at　lower　sintering　temperature　(