The　relationship　between　the　spontaneous　polarization　strength　and　effective　dielectric　constant　of　BaTiO3　and　its　grain-size　has　been　calculated　based　on　landau-ginzburg-devonshire　(LGD)　phenomenological　theory　in　this　paper.　Lich,　MGT,　EMT　and　J-S　theoretical　equations　were　used　to　simulate　the　effective　dielectric　constants　of　BaTiO3/Epoxy　composite　in　the　range　of　1　nm　to　1000　nm.　The　results　show　that　the　single　component　specimen　of　BaTiO3　with　grain　size　of　300　nm　has　a　high　effective　dielectric　constant　at　room　temperature,　and　J-S　equation　could　be　well　fitted　to　the　experimental　data　at　low　volume　fraction　(10%~40%)　when　it　was　composited　with　Epoxy.