The poly(vinyl chloride) was modified by polyhedral oligomeric silsesquioxane with 3-chloropropyl groups (CP–POSS) by POSS concentration between 0.5 and 5 wt%. Two methods of POSS incorporation were applied, i.e. direct solid powders mixing or addition of POSS to PVC solution in THF. The final samples were produced by melt mixing in the Brabender mixer chamber. By means of the glass transition temperature (Tg) determination the influence of a low contain of CP–POSS on the effect of plasticization of the PVC compounds was investigated. Depending on the measurements method (dynamic mechanical analysis DMA and differential scanning calorimetry DSC) and on the DMA measurement frequency various values of the Tg were observed. It was found that CP–POSS may act as a plasticizer, leading to certain lowering of the glass transition temperature of poly(vinyl chloride).