The decolorization of Orange II fit the first order kinetic model, C=C0 exp(−kobst),where C is the dye concentration at time t, C0 is the initial dye concentration, and kobs is the observed first order decolorization rate constant. The concentrations of the sample can be calculated from the measured absorbance values using Beer-Lambert Law. During the decolorization of Orange II by 10 mg/L of nZVI for various initial pH values, the observed first order rate constant was inversely proportional to the initial pH, except for pH 2. The effect of pH was significant on the reduction of Orange II.