Accelerated thermal ageing test on nanocomposites of polyvinyl chloride (PVC/SiO₂) is conducted at constant temperatures of 110 °C and 140 °C. Accordingly, different PVC/SiO₂ nanocomposite samples with different silicon dioxide (SiO₂) filler loading are prepared. The preparation of the samples was carried out based on the solution casting technique. The loading concentrations of SiO₂ in the investigating samples are 0, 1, 2.5, 5 and 7.5 wt%. The breakdown strength of the prepared PVC/SiO₂ nanocomposite samples is evaluated before and after the thermal ageing test. The evaluation of breakdown strength is carried out based on American Society for Testing and Materials (ASTM) standard. Also, dielectric spectroscopy, which includes dielectric constant (έ) and tangent loss (tan δ) of nanocomposite, is performed before and after the thermal ageing test. Also, the mass losses during thermal ageing of all samples are evaluated. The results show that the breakdown strength, tan δ, and the dielectric constant (έ) of pure PVC and PVC/SiO₂ nanocomposites are significantly affected by the thermal ageing test. Adding a small amount of SiO₂ nanoparticles to PVC leads to improvement in its dielectric properties before and after the thermal ageing process as compared to the pure PVC. The mass loss of PVC/SiO₂ nanocomposite during thermal ageing is less than that of pure PVC and confirms the obtained result from breakdown strength.