Acid‐converted cornstarch was subjected to a pretreatment of cationization using N‐(3‐chloro‐2‐hydroxypropyl) trimethylammonium chloride to introduce 3‐(trimethylammonium chloride)‐2‐hydroxypropyl (TMACHP) substituents onto starch chains. Then, the quaternized starch was further modified via a graft copolymerization of the starch with acrylic acid in aqueous medium using Fe2+‐H2O2 initiator. The investigation was carried out to reveal the influence of TMACHP substituents on the graft copolymerization and sizing properties. The graft copolymerization was evaluated with grafting efficiency, grafting ratio, and conversion of monomer to polymer while the sizing properties considered included paste viscosity, adhesions to fibers, film performances, mechanical properties of sized yarns, and desizability. It was found that the TMACHP substituents introduced was able to increase the grafting efficiency and ratio of the copolymerization. In addition, the quaternized starch‐g‐poly(acrylic acid) (QS‐g‐PAA) was superior to starch‐g‐poly(acrylic acid) (S‐g‐PAA) in the adhesions. The cationization was capable of toughening starch film due to significant increase in breaking elongation, work‐to‐break, and bending endurance of the film. Moreover, the mechanical properties of the yarns sized with QS‐g‐PAA were better than those with S‐g‐PAA. Furthermore, the QS‐g‐PAA was stable in paste viscosity and desizable from sized yarns. A low level of the quaternization could be adopted to improve the grafting efficiency and sizing properties.