In the present work, a mild size-control method for thiol-protected Au NPs was developed. When thiol-protected Au NIPS were reacted with tetra-n-octylammonium bromide and trifluoroacetic acid, the size of Au NPs was tuned in the range from 2 to 6 nm at room temperature. Larger Au NPs could be obtained by mild heating at 55 degrees C. This size evolution method assisted by proton acid and bromide anion is a first example of the size tuning of thiol-protected Au NPs at room temperature. Because of the mild reactivity of acid and bromide ions to organic molecules, various thiol derivatives sensitive to heat, base, and reducing agent can be utilized for this method.