Biodegradable chitosan-based nanospheres were prepared by the one-step polymerization Of L-phenylalanine-N-carboxylic anhydride initiated by water-soluble chitosan. The obtained chitosan-g-poly(phenylalanine) nanospheres with a diameter of 220-290 nm showed a strong positive charge, and thus it will be valuable as novel biodegradable cationic nanosphere.