The thermodynamic and mechanical properties of the pure PET, PET/silica nanocomposite and PET/hydroxylated silica nanocomposite were examined by molecular simulation.

The mobility of polymer increase with the addition of silica.

Fractional free volume and many kinds of energy of polymer decreases with the addition of silica.

The glass transition temperature of PET/silica nanocomposite is smaller than that of the pure PET by the addition of silica, but the glass transition temperature of PET/modified silica nanocomposite is higher than that of the pure PET by the addition of modified silica.

The addition of silica can improve the mechanical properties of nanocomposite.