The effect of filler content on the electrical and optical properties of polystyrene filled with Mg powders has been investigated. For this purpose, PS samples with Mg additive were prepared with different percentages of Mg and with different thicknesses. The experimental results showed that the conductivity of such composites increased suddenly by several orders of magnitude at a critical weight concentration. The electrical conductivity changed with increasing temperature. Furthermore, the activation energy changed with increasing filler concentration. The dielectric constant, dielectric loss and electrical conductivity changed with the concentration of the filler and with the frequency of the applied electrical field. The absorption coefficient, extinction coefficient, and energy gap of the indirect allowed and forbidden transitions, the real and imaginary dielectric constants, and the optical conductivity were determined.