Disclosed is an excellent composition for nucleic acid delivery, which is reduced in cytotoxicity and has high nucleic acid introduction efficiency and high gene expression efficiency. Specifically disclosed is a composition for nucleic acid delivery, which contains: a block copolymer that has a non-charged hydrophilic polymer segment and a cationic polymer segment a cationic polymer and a nucleic acid. The mole percentage (B/H ratio) of the cationic groups contained in the block copolymer relative to the total cationic groups contained in the block copolymer and the cationic polymer is 25-90%.
