Two cationic polyacrylamides, PAA and PAD, were synthesized for sludge dewatering. The advanced instruments such as MAR, FTIR, and SEM were used to characterize the two copolymers. Their hydrophobic association properties in water were investigated by viscosimetry as well as the dewatering performance studied by the sludge dewatering experiment. The results showed that the optimum conditions for preparation of PAA were that the initiator concentration, urea concentration, molar ratio of AODBAC to AM, and irradiation time were 0.3 parts per thousand, 1.0%, 10:90, and 60 min, respectively. Also, it was found that PAA had a stronger hydrophobic interaction with a lower intrinsic viscosity and longer dissolution time as well as a better dewatering performance. Furthermore, the charge neutralization and bridging effects were found to contribute much to the sludge dewatering by PAA in which the dewatering performance was able to be enhanced further by the hydrophobic interaction.