One of the challenges in wastewater treatment is the low efficiency in decoloring dyeing wastewater. Chicken feather, as a waste material, has a great potential in decoloring the dyeing wastewater. In this study, a lab synthesized dyeing wastewater prepared with acid blue-A dye was treated with a chicken feather keratin-based composite decolorant KA (keratin agent) using batch decoloration techniques. A modified KA (MKA) was also developed to improve the decoloration efficiency. The decoloration performance of the two decolorants was then evaluated in terms of decoloring rate, at various decolorant dosages, pH, reaction temperature and time. Under optimal conditions, the decoloration rates of the KA and MKA in treating the dyeing wastewater were 91.8% and 94.3%, respectively. IR and TEM results indicated that the KA and MKA decolorants removed the dye stuff from the dyeing wastewater by physical adsorption as well as chemical reactions.