A biodegradable cationic polymer is disclosed. The biodegradable cationic polymer of the invention has amino groups in the backbone and side chains, self-assembles cationic complexes with nucleic acids, and delivers nucleic acids into a cell by endocytosis. The biodegradable cationic polymer of the invention also has very low cytotoxicity. Methods of making and using the biodegradable cationic polymer are further disclosed.