The protonation of nitric acid by a proton has been studied by ab initio direct molecular dynamics (MD) simulation. Energies of important structures on the potential surface were calculated by the G2MP2 method. Four intermediates and four transition state structures have been identified. All four reactive intermediates were found to play significant roles in the mechanism of reaction, but the dominant channel is HNO3 + H+ --> NO2- + H2O. Reaction energies for reaction to form each of the possible products range from -150 to -180 kcal/mol.