Modem technology introduced reinforced plastics in various applications in aerospace fields. The plastic materials in these fields certainly need different additives to improve their physical and mechanical properties. A series of polyesters resins having different structures and molecular weights have been prepared. The molecular weights of the prepared polyesters were determined via end group analysis. These polyesters were blended with dioctyl phthalate (DOP) and the efficiencies of these blends as plasticizers for PVC were evaluated. It was proved that some of the prepared aliphatic polyesters improve the action of the traditional low molecular weight plasticizer. This conclusion was attained by monitoring the mechanical properties (tensile - of elongation - hardness) throughout a whole year.