A　CoCO3-added　Pb((Zn1/3Nb2/3)(0.20)(Zr0.50Ti0.50)(0.80))O-3　(0.2PZN-0.8PZT)　system　was　prepared　and　investigated.　The　results　reveal　that　Co　ions　are　present　in　the　mixed　valence　form　of　+2　and　+3,　and　their　relative　mole　ratio　depends　on　doping　content.　Co　doping　induces　a　phase　transformation　from　the　MPB　to　the　tetragonal　phase　side,　and　the　corresponding　domain　size　increases　accordingly.　The　solubility　limit　of　Co　ions　in　the　perovskite　matrix　is　near　0.2　wt.%　in　CoCO3　form.　Below　the　solubility　limit,　Co　ions　enter　the　B　sites　of　the　oxygenic　octahedral　center,　and　the　charge-compensating　oxygen　vacancies　accelerate　mass　transport,　which　assists　in　the　densification　of　the　specimens.　Above　the　solubility　limit,　excess　Co　ions　gather　in　the　grain　boundaries　and　triple　junctions,　which　facilitate　the　formation　of　a　liquid　phase　with　excess　PbO　and　lead　to　remarkable　grain　growth.　The　resulting　size　effect　plays　a　dominant　role　in　improving　the　piezoelectric　properties,　which　compensates　for　the　hardening　effect　of　acceptor　doping.　The　optimum　piezoelectric　parameters　are　obtained　at　0.8　wt.%　CoCO3-doped　0.2PZN-0.8PZT　system:　d(33)　=　310　pC　N-1,　k(p)　=　0.66,　and　epsilon(max)　=　14,600.　(C)　2012　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.