This study was conducted to illuminate the effectiveness of dual-conditioning compared with that of conventional mono-conditioning. In this method, both cationic and anionic polymer were successively dosed into water treatment plant sludge for dewaterability improvement. As results, sludge dewaterability was more enhanced by dual-conditioning method than any other mono-conditioning method using cationic polymer as well as anionic polymer. As sludge pH increased using NaOH. sludge properties including turbidity, viscosity, particle size and zeta-potential were more deteriorated. This made poor dewaterability and also increased optimal polymer dosage. Sludge properties, especially zeta-potential affected the effectiveness of dual-conditioning as well. In case zeta potential became under about -20㎷, dewatering performance by dual-conditioning method was similar to that by mono-conditioning method even though dosage of dual polymer was more lower compared with that of mono polymer. Therefore, dual-conditioning method adapted in this study might be less expensive than conventional mono-polymer conditioning method as depended on zeta-potential.