ZnO-B2O3　(ZB)　doped　CaCu3Ti4O12　(CCTO)　ceramics　were　prepared　by　solid-state　reaction,　from　the　mixture　of　ZB　and　CCTO　powder　calcined　at　1　100°C.　The　effects　of　ZB　on　the　microstructure　and　dielectric　properties　of　CCTO　ceramics　were　investigated.　And　the　dielectric　mechanism　with　giant　dielectric　constant　was　also　analyzed.　The　results　show　that　CCTO　ceramics　with　a　small　amount　of　ZB　additive　(w≤2%,　mass　fraction)　is　of　single　CCTO　phase　with　BCC　perovskite　structure.　Zn2TiO4　phase　is　detected　when　w　is　above　2%.　For　the　CCTO　ceramics　with　w=0.5%　and　10%,　the　dielectric　constant　is　enhanced　obviously,　while　the　dielectric　loss　is　higher　than　that　of　CCTO.　On　the　other　hand,　for　the　CCTO　ceramics　with　1%-5%　ZB　additions,　the　dielectric　constant　is　nearly　independent　of　ZB　content　and　frequency.　And　they　exhibit　low　dielectric　loss　and　good　thermal　stability.　Especially,　CCTO　ceramics　with　2%　ZB　exhibit　the　excellent　dielectric　properties:　relative　dielectric　constant　εr=336,　dielectric　loss　tan　δ=0.018,　temperature　coefficient　of　dielectric　constant　τε=-1.5　×　10-5　°C-1.　The　impedance　spectra　of　the　ZB-doped　CCTO　ceramics　indicate　that　CCTO　ceramics　consist　of　semiconducting　grain　boundaries　and　insulating　grains,　which　results　in　the　giant　dielectric　constant　of　CCTO　ceramics.