A brush-shaped polymer PHEMA-g-(PEI-b-PEG) with poly(2-hydroxyethyl methacrylate) (PHEMA) backbone and linear poly(ethylenimine)-b-poly(ethylene glycol) (PEI-b-PEG) side chains was synthesized and evaluated as a vector for potential cancer gene therapy. The characterizations by (1)H NMR and laser light scattering demonstrated the brush structure of the polymer. PHEMA-g-(PEI-b-PEG) was much less cytotoxic when compared with branched poly(ethylenimine) with M(w) of 25 kDa. The capacity of plasmid DNA condensation by PHEMA-g-(PEI-b-PEG) was demonstrated by gel retardation assay, and they formed nanosized complexes with surface zeta potential around 20 mV at N/P ratios higher than 5:1. The complexes of PHEMA-g-(PEI-b-PEG) with plasmid DNA were more efficiently internalized by BT474 cells in comparison with the complexes of PEI25K, leading to higher gene transfection in cells. Further investigation using complexes of PHEMA-g-(PEI-b-PEG) with plasmid DNA encoding wild-type p53 gene showed its potential as a carrier for cancer gene therapy. The complexes of PHEMA-g-(PEI-b-PEG) successfully induced elevated wild-type p53 expression in BT474 cells and led to enhanced apoptosis of BT474 cells. Transfection of wild-type p53 using the complexes also significantly increased the sensitivity of BT474 cells to doxorubicin chemotherapy, suggesting the potential of this carrier in cancer gene therapy.