Polyvinyl chloride (PVC) is widely used in high and medium voltage cables insulation because of its low dielectric losses and its ability to improve cables properties in high temperatures. This paper aims to improve PVC electrical properties (dielectric strength) for high voltage and medium voltage cables in respect of mechanical characteristics by adding inorganic filler. Blends of PVC with inorganic filler : Silica (Sio2) were prepared with 10% ,20% , 30% and 40% by weight percentages concentration. The dielectric strength of the blends was tested in several thermal conditions such as (0 , 25 ,65 and 125C). Also dielectric strength of blends was tested after being immersed in water for 24 hours, 7 days and 10 day. Tensile strength was applied to investigate the mechanical properties of PVC after adding silica filler with 10% , 20% , 30% and 40% concentration percentages. Soft program (Curve fitting) is used to select the most proper equation between dielectric strength values under each condition and different percentages of Silica filler also between tensile strength and the same percentages of Silica filler. Grey Wolf Optimizer (GWO) was applied to find the best suitable percentages of Silica filler to obtain the best optimal values of dielectric strength and tensile strength .