The bulk poly(ethylene terephthalate) (PET) and the filament PET samples filled with different amounts of carbon black and constant amounts of TiO₂ were prepared in order to investigate the crystallization behavior. mechanical properties and thermal properties of PET. For the bulk sample, as carbon black content in PET increases, crystallinity, crystallization rate and glass transition temperature increased, whereas melting point temperature remained constant. The filament in melange yarn with higher concentration of carbon black showed higher glass transition temperature than those with lower concentration of carbon black. However, there were no change in melting point temperature and particularly in crystallinity, which might be attributed to the thunderbolt phenomenon. Increase of carbon black content in filament also resulted in increase of elongation and decrease of tenacity. apparent Young''s modulus, yield point stress and initial tensile load. Furthermore the variation of yarn toughness with carbon black content was also investigated.