Strength properties and water repellency of cotton stalk fibers esterified with propionic anhydride that are used as reinforcing of novolac resin were greatly improved than untreated. Adding aluminum oxide to the prepared composite improved bending strength and enhanced water repellant. The alternating current conductivity and dielectric constant were greatly enhanced at different frequencies by adding aluminum oxide. The more percent of aluminum oxide, the more value of alternating current conductivity and lower value of dielectric constant were obtained at different frequencies. Infrared and X-ray diffraction as well as scanning electron microscope were studied.