The present invention provides a method for preparing a nanoporous ultra-low dielectric thin film including a high-temperature ozone treatment and nanoporous ultra-low dielectric thin film prepared by the same method. The method includes preparing a mixture of an organic silicate matrix-containing solution and a reactive porogen-containing solution; coating the mixture on a substrate to form a thin film; and heating the thin film with an ozone treatment. The prepared nanoporous ultra-low dielectric thin film could have a dielectric constant of about 2.3 or less and a mechanical strength of about 10 GPa or more by improving a pore size and a distribution of pores in the thin film by performing an ozone treatment with high temperature and optimization of the ozone treatment temperature.