An excellent nucleic acid delivery composition is provided which has reduced cytotoxicity and improved nucleic acid introduction efficiency and gene expression efficiency. The composition comprises: a block copolymer having an uncharged hydrophilic polymer segment and a cationic polymer segment a cationic polymer and a nucleic acid, wherein the mol percentage (B/H ratio) of the cationic groups of the block copolymer to the total cationic groups of the block copolymer and the cationic polymer is between 25% and 90%.
