Oxazolidinones are important biologically active heterocyclic compounds and it can be synthesized from the cascade organocatalysis reaction of stable sulfur ylides and niroolefins catalyzed by thiourea and N,N-dimethylaminopyridine (DMAP)[1-3].The reaction is theoretically studied using density functional theory (DFT) in the presence of the continuum solvation model (SMD) with chloroform as the solvent.It is shown that the non-enantioselective rate-determining step is the addition reaction of sulfur ylide and nitroolefin and there are two competing reaction channels in the addition reaction path.One is that the nitro cyclopropane is generated first and then converted into isoxazoline N-oxide by a DMAP-catalyzed rearrangement.The other is the isoxazoline N-oxide intermediate is directly generated.DMAP plays an important role in the reaction as a nucleophilic catalyst,especially in the step of the isomerization reaction of nitrogen oxides on isoxazoline.It is the first time that the Hofmann rearrangement is confirmed by theoretical calculations.The M06-2X[4-6] exchange correlation functional is employed in our calculations and our results are good agreement with the experiment.