Acid-modified multi-walled carbon nanotubes (a-MWNTs) reinforced poly(ethylene 2,6-naphthalate) nanocomposites were prepared by in situ polymerization. Non-isothermal crystallization behavior of in situ polymerized PEN/a-MWNT nanocomposites was studied at various cooling rates. Non-isothermal crystallization kinetics were analyzed by using Avrami, Ozawa, and Ziabicki equations. The PEN/a-MWNT nanocomposites showed higher crystallization rate and temperature than PEN, which was thought to be due to the incorporation of MWNT as a nucleating agent.