Polymer nanocomposites based on multiwall carbon nanotube (MWCNT) and poly(ethylene terephthalate) (PET) were prepared by a melt blending process in a twin-screw extruder. The non-isothermal crystallization behavior of the PET/MWCNT nanocomposites was investigated by employing the Ozawa, the Avrami, and the combined method. The non-iso-thermal crystallization behavior of the PET/MWCNT nanocomposites significantly depends on cooling rate and MWCNT content. The incorporation of MWCNT accelerates the nucleation and crystal growth mechanisms of PET, and this effect being more appreciable at lower MWCNT content. The combined Avrami and Ozawa method was found to be more effective in describing the non-isothermal crystallization process of PET/MWCNT nanocomposites. The nucleation activity of PET/MWCNT nanocomposites was increased with the incorporation of MWCNT. The activation energy for crystallization of PET/MWCNT nanocomposites was higher than that of PET. The incorporation of MWCNT improved the thermal stability and mechanical properties of PET/MWCNT nanocomposites.