Poly(ethylene terephthalate) (PET) nanocomposites containing multi-walled carbon nanotubes (MWNT) of different diameters and aspect ratios were prepared by coagulation method. The diameter and aspect ratio MWNT with the same mass percent shows substantial effects on crystallization behavior and electrical conductivity of PET. FESEM observations show that the diameter of MWNT could affect their interactions with the surrounding PET and the thickness of interfacial PET layer. High aspect ratio of MWNT can facilitate the overlapping of MWNT and the formation of conductive networks. The electrical performance of PET/MWNT nanocomposites can be tuned by controlling the thickness of interfacial polymer layer and the aspect ratios of MWNT. The smallest diameter MWNT possesses the strongest interfacial interactions with the PET chains, leading to the most noticeable increase in nucleating effect and crystal growth rate. Moreover, MWNT with high aspect ratio can facilitate the growth and development of PET crystals due to the greater nucleating efficiency and less restricted effect to PET crystallization. However, MWNT with small aspect ratio owns much more nucleation sites than MWNT with high aspect ratio at the same loadings, which may enhance nucleation density and nucleation rate of PET crystallization, but result in slower PET crystal growth and broader crystal size distributions. POLYM. ENG. SCI., 56:408-417, 2016. (c) 2016 Society of Plastics Engineers