The mechanical properties of composites of granular starch and low density polyethylene (PE) have been studied as functions of starch volume fraction phi, granule size, and presence of compatibilizer. Property-volume fraction relationships were interpreted using various theories of composite properties. The dependence of elongation (epsilon similar to phi(1/3)) and tensile strength (sigma similar to phi(2/3)) agree with theoretical predictions, although the proportionality constants are less negative than theoretic al values. The addition of compatibilizer (ethylene-co-acrylic acid copolymer, EAA) did not significantly affect the elongation or tensile strength, but significantly increased the composite tensile modulus. The cornstarch/PE moduli could be described by the Kerner or Halpin-Tsai equations. Analysis of the composite moduli data using the Halpin-Tsai equation allowed the estimation of the modulus of granular starch. The value obtained, 15 GPa, is considerably greater than most unfilled synthetic polymers of commercial importance, but significantly lower than the modulus of cellulose. It is also greater than a previously reported value of 2.7 GPa.