The effects of the -COOH groups on polystyrene particle surface on the heterogeneous coating of hydrous alumina in an aqueous process were studied. Pure polystyrene particles and particles loaded with different densities of -COOH groups were used in the hydrous alumina coating process. The particles were synthesized by the emulsifier-free emulsion polymerization of pure styrene, and copolymerization of styrene and methacrylic acid (MAA), with potassium persulfate (K2S2O8) as initiator. The amount of the -COOH groups on the copolymerized particle surface was determined by conductometric titration. High-resolution transmission electron microscopy (HRTEM) images showed that, when the density of -COOH groups loaded on the particle surfaces was high enough, the particles could be easily heterogeneously coated with hydrous alumina; when it was too low, there was much homogeneous nucleation on the coated particles and in the bulk. In the absence of -COOH groups, i.e., pure polystyrene, the particles could hardly be coated under the same conditions. It is inferred that the precipitation and reaction occurred at the sites of -COOH groups on the particle surfaces in the coating process.