A copolymer of glycidyl methacrylate and styrene (PGS) was synthesized by emulsion polymerization. The effect of the polymer on paper dry- and wet-strength improvement was investigated. The polymer contains phenyl groups that provide hydrophobic character to fibers so as to improve paper wet performance, and glycidyl groups that react with nucleophilic groups in fibers and other polymers to increase paper strength. Results showed that other polymers, such as polyallylamine, polyacrylic acid, and modified polyacrylamide, could be used together with PGS for paper-strength improvement. Furthermore, three kinds of partially carboxymethylated pulps (CMP) were prepared and used for PGS treatment. Compared with original pulp sheet, the sheets containing CMPs were strengthened greatly both in dry and in wet strength after PGS treatment. It was confirmed that higher CMP content got higher strength improvement at the same addition level of PGS. The results showed that carboxyl groups in pulp participate in the reaction with PGS and the cocrosslinkings between fibers and polymers were formed for improving paper strength.