Root oxidizing ability is the requisite for the formation of iron plaque on roots surface. Fe3+ is a major form in the iron plaque. Therefore, according to the quantity of iron plaque on roots surface, this paper explored a novel method for quantifying the oxidizing capacity of rice roots. First of all, added quantitative ferrous sulfate (Fe2+) into the culture solution, rice roots would oxidized parts of Fe2+ to Fe3+ after 1 d, remaining Fe2+ in the culture solution was titrated with H2O2, the titration value was A. Then, H2O2 titrated Fe2+ of culture solution without cultivation rice seedlings, the titration value was B. The difference of H2O2 titration value (B-A) was named Root oxidizing ability. This method required: Fe2+ concentration was more than 0.8 mmol/L, H2O2 consumption was proportional to Fe2+ (R2 = 0.999 2). During the determining process, the culture solution was added 1, 10-Phenanthroline monohydrate as colour-display reagent firstly, then added phosphoric acid. This method could quantitatively determine oxidizing capacity of rice roots which were not damaged.