Polycarboxylate copolymers (PCs) with poly(ethylene oxide) (PEO) side chain were synthesized using first an esterification reaction technique and second a free radical polymerization in aqueous solution. The resultant PCs have been characterized by Fourier transform infrared (FTIR) spectrum and gel permeation chromatography (GPC). The molecular structural parameters of PCs were determined from GPC and ion chromatography (IC). The dispersibility of PCs having different degree of polymerization (DP) of PEO side chain was investigated in cementitious system. Also, the mechanism of dispersion was discussed via the respect of electrostatic repulsion and steric hindrance effect. The results show that the PCs possessing an appropriate DP of PEO side chain can exhibit a higher dispersibility.