Multi-walled carbon nanotubes (MWNT) reinforced poly(ethylene 2,6-naphthalate) (PEN) nanocomposites were prepared by in situ polymerization. To improve the dispersibility of nanotubes, MWNT was treated with acid and consequently carboxylic acid groups were introduced on the surface of MWNT. The carboxylic acid groups formed on the surface of MWNT were confirmed by XPS analysis. Acid-modified MWNT showed a good dispersiblity in ethylene glycol. MWNT dispersion in ethylene glycol was introduced into the bis-hydroxyethylnaphthalate (BHEN) melt at the end of the ester-interchange reaction stage of poly(ethylene 2,6-naphthalate) synthesis. Crystallinity and crystallization temperatures were increased with MWNT content, which implied that MWNT could aid PEN to crystallize more fast and much more.