PET/Cloisite 30B (C30B) nanocomposites containing 2 and 3.5 wt % of the organoclay were prepared using two different twin-screw extrusion processes: conventional melt-mixing and water-assisted melt-mixing. The reduction of the molecular weight of the PET matrix, caused by hydrolysis during the water-assisted extrusion, was compensated by subsequent solid-state polymerization (SSP). XRD and TEM analyses show intercalated/exfoliated morphology in all PET/C30B nanocomposites, with a higher degree of intercalation and delamination for the water-assisted process. The 2 values of XRD peaks of C30B in PET nanocomposites were not affected by clay concentration. However, the intensity of the peaks in the nanocomposites containing 2 wt% C30B was smaller than for the higher C30B concentration. This suggests more agglomerated structure for the higher organoclay concentration, which is also confirmed by TEM images. Dispersion and distribution uniformity of C30B were better in the nanocomposites prepared by water-assisted extrusion. The barrier property of the nanocomposites containing 3.5 wt % of C30B was 40 % better than that of the neat PET. At 3.5 wt % of C30B, the tensile modulus was about 35% higher after SSP, but the elongation at break decreased significantly.