The kinetics and mechanisms for the reaction of the Criegee intermediate CH2OO with HNO3 and the unimolecular decomposition of its reaction product CH2(O)NO3 are important in atmospheric chemistry. The potential-energy profile of the reactions predicted with the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ method shows that the initial association yields a prereaction complex that isomerizes by H migration to yield excited intermediate nitrooxymethyl hydroperoxide NO3CH2OOH* with internal energy similar to 44 kcal mol(-1). A fragmentation of this excited intermediate produces CH2(O)NO3 + OH with its transition state located 5.0 kcal mor(-1) below that of the reactants. Further decomposition of CH2(O)NO3 produces HCO + HNO3, forming a catalytic cycle for destruction of CH2OO by HNO3. The rate coefficients and product-branching ratios were calculated in the temperature range 250-700 K at pressure 20-760 Torr (N-2) using the variational-transition-state and Rice-Ramsperger-Kassel-Marcus (RRKM) theories. The predicted total rate coefficient for reaction CH2OO + HNO3 at 295 K, 5.1 x 10(-10) cm(3) molecule(-1) s(-1), agrees satisfactorily with the experimental value, (5.4 +/- 1.0) X 10(-10) cm(3) molecule(-1) s(-1). The predicted branching ratios at 295 K are 0.21 for the formation of NO3CH2OOH and 0.79 for CH2(O)NO3 + OH at a pressure of 40 Torr (N-2), and 0.79 for the formation of NO3CH2OOH and 0.21 for CH2(O)NO3 + OH at 760 Torr (N-2). This new catalytic conversion of CH2OO to HCO + OH by HNO3 might have significant impact on atmospheric chemistry.