In order to enhance the pretreatment effect of dyeing wastewater, comparison experiments of coagulation micro-bubble floatation and coagulation conventional air bubble flotation were carried out. The results demonstrated that coagulation micro-bubble flotation significantly reduced coagulant doses and enhanced the pretreatment rate, because the usage of micro-bubble generator produces fog-like microbubbles with diameters in tens of micrometer, which has longer preservation time in the water. In addition, the micro-bubbles could reach a higher oxygen transfer rate: the total mass-transfer coefficient of micro-bubbles was 1.1754 min(-1), and that of conventional air bubbles was of 0.7535 min(-1). Under the same conditions, the removal efficiencies of COD, color and oil for the micro-bubble floatation increased 30%, 110% and 40% respectively, compared with the conventional air bubble flotation. Furthermore, the wastewater biodegradabilities were increased from 0.290 to 0.363 after coagulation micro-bubble flotation and to 0.301 after coagulation conventional air bubble flotation. The application of the micro-bubble technology in coagulation processes may provide an efficient and cost-effective approach for the wastewater treatment. (C) 2010 Elsevier By. All rights reserved.