Cationic and anionic polymers with the same molecular weight were used as sludge conditioning reagent to study the polymer charge effect on alum sludge dewatering characteristics. On the basis of capillary suction time, bound water content, and zeta-potential measurement in this study, the function of charge neutralization appears to be an important consideration in the sludge dewaterability and moisture content. Using the dilatometric technique to measure the bound water content, we found that alum sludge conditioned by cationic polymer presented more significant variation in the bound water content than sludge subject to anionic conditioning. We hypothesise that the mechanism of charge neutralization caused much water depletion by replacement and exclusion in sludge and results in the decrease of bound water within the cationic polymer conditioning. An insignificant bound water variation found during the anionic conditioning resulted from the lack of charge neutralization. A lighter and looser floc structure is also be found in the anionic conditioned sludge after the analysis of wet floc density and fractal dimension. Such open floc structure can be attributed to the lack of charge neutralization as the flocs aggregate with anionic polymer.