Uniform-sized amino-modified silica nanoparticles have been prepared by the controlled synchronous hydrolysis of tetraethoxysilane and N-(β-amimoethyl)-γ-aminopropyltriethoxysilane in water nanodroplet of the wa-ter-in-oil microemulsion. These nanoparticles display positive charge potential at definited pH. This is due to the presence of amino groups on the surface of the nanoparticles. Nanoparticles-plasmid DNA complexes can easily form through electrostatical binding between the positive charges of the amino-modified silica nanoparticles and the negative charges of the plasmid DNA. The complexes can be also dissociated under alkaline pH or high ionic strength conditions. And enzymatic digestion of the plasmid DNA is almost inhibited by these nanoparticles complexes. A novel non-viral gene carrier based on the amino-modified silica nanoparticles is proposed under the combination of nanotechnology, biotechnology and gene engineering technology. The plasmid DNA can successfully cross various systemic barriers to COS-7 cells as well as mediate high expression of Green Fluorescence Protein (GFP) gene in cells by use of this novel gene carrier.