A　series　of　poly(ethylene　terephthalate)　(PET)　nanocomposites　were　prepared　by　in　situ　polymerization　containing　different　amounts　of　surface　treated　calcium　carbonate　(CaCO3).　As　verified　by　SEM　micrographs,　the　dispersion　of　CaCO3　nanoparticles　into　the　PET　matrix　was　homogeneous　when　the　content　of　CaCO3　was　below　5　wt.%,　however,　some　aggregates　existed　at　higher　filler　content.　DSC　study　results　showed　that　the　crystallization　became　faster,　and　there　was,　also,　a　shifting　of　temperature　of　the　crystallization　peak　to　higher　value　by　the　addition　of　CaCO3　till　5　wt.%,　this　being　evidence　that　CaCO3　can　act　as　nucleating　agent.　From　TG　curves　it　was　concluded　that　PET　and　the　samples　with　different　content　of　CaCO3　nanoparticles　presented　good　thermal　stabilities,　since　no　remarkable　mass　loss　occurred　up　to　360　degrees　C　(＜0.5%).　Moreover,　from　the　variations　of　the　activation　energies,　calculated　from　the　TG　curves,　it　was　clear　that　nanocomposites　containing　CaCO3　had　a　higher　activation　energy　compared　to　the　pure　PET,　indicating　that　the　addition　of　the　nanoparticles　could　slightly　increased　the　thermal　stability　of　the　matrix.　(C)　2013　Elsevier　B.V.　All　rights　reserved.