tIn the current investigation, nanocomposites of poly(ethylene terephthalate) (PET) with different typesof organo-clays were produced using the melt mixing technique. Two types of commercial inorganicclays (Laponite-synthetic hectorite and Kunipia-montmorillonite) were studied after cation-exchangewith hexadecyltrimethylammonium bromide (CTAB) while two commercial organo-modified mont-morillonite clays (Nanomer I.30E modified with primary octadecylammonium ions and Cloisite 10Amodified with quaternary dimethyl benzyl hydrogenated-tallow ammonium ions) were also investi-gated. The structure of the nanocomposites was studied by X-ray diffraction measurements. A detailedcrystallization analysis was carried out by means of both isothermal and non-isothermal (melt and cold)measurements. All data were analyzed using the simple Avrami equation along with advanced isocon-versional methods. The nucleation activity of the filler was investigated in every case. Lauritzen–Hoffmananalysis was employed to isothermal data to estimate the nucleation parameters. From all these mea-surements it was found that the organo-clay I.30E induces the higher crystallization rates and loweractivation energy and is more effective regarding the PET crystallization compared to the other types oforgano-clays. The I.30E organo-clay nanocomposite exhibited also the higher immobilized amorphousfraction and the higher nucleation parameter Kgin the Lauritzen–Hoffman analysis. This is due to its betterdispersion and exfoliation of the clay nanolayers into the PET matrix, compared to the other organoclays.