Ab initio molecular orbital calculations at the G2M(cc3) level of theory were used to study the mechanism for the OH+HNO3 reaction. Intrinsic reaction coordinate analyses was used to find and confirm four complexes and four transition states. Six membered ring complex formed by hydrogen bonding of HNO3 with OH radical was found to be stable by 8.1Kcal/mol. Insignificant to atmospheric chemistry, direct abstraction process producing H2O2 and NO2 was predicted to have a barrier of 24.4 Kcal/mol. Below room temperature reaction was found to have strong pressure and tunneling effects. Below room temperature, the rate constants for the decay of OH and OD (in OD+DNO3) evaluated by kinetic mode.