With more H+ in the solution, the reaction occurred more easily and the reaction constant was high. However, when the initial pH was 2, the decolorization rate decreased. According to Fenton's chemistry, if reaction is carried out in a high pH condition, the iron forms a precipitate, Fe(OH)3, which decomposes H2O2 to O2. Also, if pH is too low, Fe(III) is reconverted to Fe(II). Both cases lead to reduced reaction rate by decreasing the starting reagents [5]. The results of this study indicate that the efficiency of decolorization of the azo dye by nZVI increased with a decrease in the systempH. This findingwas consistent with the results reported by several researchers [11,14] using microsized zero-valent iron. During the reduction of Orange II with nZVI, the pH increased from 3 to 3.9, 5 to 5.5, and 7 to 7.3. After the reduction reactions were completed, pH became constant. The reduction of Orange II by nZVI consumed H+ ions, which resulted in an increase in the pH during the early stages of the reaction.