Octenylsuccinylation of cornstarch had been performed to improve the properties of cornstarch for sizing polyester/cotton blend spun yarns. The starch was octenylsuccinylated with octenylsuccinic anhydride to different levels of substitution in order to find the effect of octenylsuccinylation extent on the properties. The influences of starch octenylsuccinylation upon paste viscosity, viscosity stability, surface tension, and adhesion to cotton or polyester fibers, performance of starch film, alone with the mechanical properties and hairiness of sized yarns were studied. It was found that the octenylsuccinylation was able to stabilize paste viscosity of starch, reduce surface tension of cooked starch paste, increase adhesion of starch to polyester fibers, ameliorate performances of starch film, improve mechanical properties of sized polyester/cotton yarns, and decrease hairiness on surface of sized yarns. Initial increase in the level of octenylsuccinylation enhanced these positive effects, while excessive level resulted in marked reduction in tensile strength of starch film and significant decrease in reaction efficiency. In addition, the modification does not raise the degree of desizing difficulty under the modification extent considered. When degree of substitution was 0.025, the octenylsuccinylation could be applied to improve the performances of cornstarch for sizing polyester/cotton yarns.