The isomerization mechanism of peroxynitrous acid (ONOOH) in the aqueous solution has been studied by using density function theory. The geometries of reactants, transition states, intermediates and products have been optimized by B3LYP/6-31G(d) basis set and verified by frequency analysis. The reaction pathway was traced by Fukui's theory of intrinsic reaction coordinate (IRC). The reaction channels were verified. The study shows that the isomerization reaction is a multi-step and multi-channel complex one, and highly exothermic. Among them the channel (1) is a principal one. The activation e...