A thermosetting resin composition due to containing a modified PPE resin as a main ingredient is suited for use in making a pregreg or a copper foil substrate, when hardened, featuring a small dielectric constant (Dk), a low dielectric dissipation factor (Df) and a high Tg as well as a high resistance to heat and flame, this outstanding result is because the modified PPE resin is formed with a novel two-dimensional hardenable structure prepared to have side-chain reactive functional groups being provided in addition to those at the terminal ends of the main chain of the PPE resin thereof.