In this study, decolourization and COD removal of Reactive Red 120, Remazol Brilliant Blue, Reactive Green 19 and Reactive Black 5 by ozonation and biological treatment were evaluated to apply for wastewater treatment containing azo dye. The performance of COD and colour removal in the single-stage ozonation-biological treatment was also compared with the multi-stage ozonation-biological treatment processes. Ozonation transforms the functional groups in azo dye to produce more biodegradable by products, which is easily removed by biological treatment. Ozonation is efficient for decolourization of Reactive Red 120, Remazol Brilliant Blue, Reactive Green 19 and Reactive Black 5 even with lower ozone dose (0.3 mg O 3 . mg dye -1 ). Contrarily, significant decreased in COD concentration was only observed within higher range of ozone doses (1.2 mg O 3 . mg dye -1 ). Higher COD removal at high ozone dose was due to complete oxidation of azo dye. The result indicated that biodegradable fraction of COD could be further oxidized and completely removed by ozonation. COD removal for Reactive Red 120 in the single stage ozonation-biological treatment and ozonation-biological treatment that repeated for 4 times were 58 % and 75 % respectively. The improvement of COD removal in the multi-stage ozonation-biological treatment was attributed to the production of biodegradable fraction of COD.