The corn starch was grafted by microwave irradiation with rheological phase reaction, to meet the sizing requirements of polyester/cotton blended yarn and avoid the disadvantages of corn starch size including brittle, hard, and size shedding. On the basis of the damping characteristics of corn starch film and the polarity of grafting monomer, the grafting effect of grafted corn starch was researched by using infrared spectroscopy, SEM, rotary viscometer, and thermomechanical analyzer. The results showed that both hydrophilic groups and hydrophobic groups were introduced in the molecular chains of grafted corn starch. The storage modulus of grafted corn starch film reached 285-315 MPa, being four times higher than that of nongrafted corn starch film. Compared with corn starch sizing, the viscosity of grafted corn starch sizing decreased 62.5%. Furthermore, through analyzing grafting law and establishing regression equation, it was indicated that the irradiation time and initiator content influenced the grafting ratio of grafted starch significantly. The grafting ratio could reach 19.57%, when the irradiation time and initiator content were selected with 3.5 min and 4.55%, respectively. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1481-1487, 2011