Inefficient release of polymer/DNA complexes from endocytic vesicles into the cytoplasm and the cytotoxic nature of cationic polymers are two of the primary causes of poor gene delivery. EG-polyurethane [poly(ethylene glycol)-PU, Poly 1], EGDM-polyurethane [poly(ethylene glycol), 2-(dimethylamino)ethylamine-PU, Poly 2], and MDEADM-polyurethane [N-methyldiethanolamine, 2-(dimethylamino)ethylamine-PU, Poly 3] were designed in this study to overcome these obstacles. The structural characteristics of polyurethanes and physicochemical properties of their formed complexes with DNA were determined to correlate their transfection efficiency. The results revealed that Poly 2 and Poly 3 could bind with plasmid DNA and yield positively charged complexes with a size required for transfection. Poly 3 showed the best in buffering capacity and its formed complexes with DNA could transfect COS-7 cells better than those of Poly 2 and Poly 1. This study reveals that the amine groups in the polymeric structure and the buffer capacity of a polymeric transfectant would affect its potential in DNA delivery. Also the size and binding properties of DNA and polymeric transfectants can be in correlation to the transfection efficiency of resulting DNA/polymer complexes.