In this report, we demonstrate a rapid and simple seeded growth method for synthesizing monodisperse, quasi-spherical, citrate-stabilized Au nanoparticles (Au NPs) via H2O2 reduction of HAuCl4. Au NPs with diameter ranging from 30 to 230 nm can be synthesized by simply adding 12 nm citrate stabilized Au NP seeds to an aqueous solution of H2O2 and HAuCl4 under ambient conditions. The diameter of the resulting Au NPs can be quantitatively controlled by the molar ratio of HAuCl4 to the Au seeds. The standard deviation of the Au NP sizes is less than 10%, and the ellipticity (ratio of major to minor axes) of the NPs is less than 1.1. Compared to existing ones, the present seeded growth approach is implemented within 1 min under ambient condition, and no unfavorable additives are involved because H2O2 can readily decompose into H2O during storage or via boiling.