A hydrophobically modified cationic flocculant, in the form of aqueous dispersion, has been successfully synthesized via dispersion copolymerization of acrylamide (AM), 2-methylacryloylxyethyl trimethyl ammonium chloride (DMC), and methyl methacrylate (MMA) in aqueous ammonium sulfate (AS) solution. The molecular structure of the prepared copolymer was characterized by nuclear magnetic resonance spectroscopy (NMR). Meanwhile, the influence of hydrophobic comonomer (MMA) on the viscosity and microstructure of this aqueous dispersion was investigated. It was found that both the viscosity and the particle size were decreased with increasing content of MMA. Thereafter, this flocculant was used to treat the oily wastewater, and its oil removal ability was assessed under various conditions. It was demonstrated that the hydrophobically modified flocculant has obvious superiority for oily wastewater treatment. POLYM. ENG. SCI., 55:1-7, 2015. (c) 2014 Society of Plastics Engineers