A nucleic acid formulation for use in gene delivery comprising a nucleic acid and an anionic polymer is disclosed. Examples of the anionic polymer includes aniionic amino acid polymer or poly-amino acid (such as poly-L-glutamic acid, poly-D-glutamic acid, poly-L-aspartic acid, poly-D-aspartic acid), poly-acrylic acid, polynucleotides, poly galacturonic acid, and poly vinyl sulfate.