Poly (ethylene terephthalate) (PET)-silica (SiO₂)/polystyrene (PS) nanocomposite films were prepared by melting PET with the core-shell SiO₂/PS nanoparticles. Differential scanning calorimetry (DSC) results showed that the crystallization temperature of PET-SiO₂/PS nanocomposite films with 2 wt.% PS-encapsulated SiO₂ nanoparticles reached 205.1 °C, 11.6 °C higher than that of PET. For crystallized PET-SiO₂/PS nanocomposite films, double melting peaks appeared in DSC curves similar to PET. Scanning electron microscopy revealed a netlike fibre morphology for the amorphous PET-SiO₂/PS nanocomposite films with 2 wt.% PS-encapsulated SiO₂ nanoparticles. The light transmittance of these amorphous PET-SiO₂/PS nanocomposite films reached 87.9%, compared to 84.2% for PET. With the increase of annealing temperature from 110 to 150 °C, the transmittance of PET-SiO₂/PS nanocomposite films decreased slowly from 69.9 to 46.9%, while their haziness increased slightly from 45.8 to 48.2%. All these phenomena are suggested to result from the strongly heterogeneous nucleation of PS-encapsulated SiO₂ nanoparticles in PET.