By varying the ratios of N-(3-chloro-2-hydroxypropl) trimethylammonium chloride and orthophosphate to starch, a series of amphoteric starch with different degree of substitution (DS) were prepared for evaluating sizing effect of amphoteric starch for cotton warps. The amphoteric starch contained quaternary ammonium and phosphate groups simultaneously, and was set to electric neutrality by varying relative quantity of anionic and cationic groups in order to prevent adverse effects of negative and positive charges. The influence of amphoteric modification of starch on the adhesion to cotton fibers was assessed by measuring tensile strength and work-to-break of slightly sized cotton roving and comparing the adhesion of amphoteric starch with those of phosphorylated one. Comparison on mechanical performances of amphoteric starch film over phosphorylated one was evaluated in terms of tensile strength, breaking extension and wear loss of starch film. The properties such as increase in tensile strength, loss in elasticity, abrasion resistance, and hairiness of cotton yarns sized with amphoteric starch were evaluated through control tests by the comparison with those of phosphorylated one. When zeta potential of amphoteric starch was set close to zero, the adhesion increased and the properties of sized yarns enhanced as the modification level increased. The amphoteric starch was evidently superior to phosphorylated one in improving the quality of sized cotton yarns. The amphoteric starch with neutral zeta potential and DS levels of 0.02-0.03 for quaternary ammonium groups and phosphate ones, respectively, could be applied to size cotton warp yarns for the improvement of yarn quality.