Experiments were conducted to investigate the decolorization of methyl orange simulated wastewater in order to assess the effectiveness and feasibility of ultrasound(US) enhanced high-purity chlorine dioxide(ClO2) oxidation process. The results showed that in ClO2/US system the decolorization rate of methyl orange was up to 96%, which was increased by 8% as compared to ClO2 treatment alone. The decolorization of methyl orange with/without ultrasonic irradiation follows apparent pseudo-first-order reaction kinetics. The apparent pseudo-first-order rate constant kapp was 0.19min-1 in the ClO2/US system, which was a little higher than 0.13min-1 of rate constant achieved in ClO2 treatment alone. It shows that ClO2/US system can be an effective technology for the decolorization of azo dyes in wastewater.