Abstract The ethylenediamine-catalyzed decarboxylation mechanism of oxaloacetic acid is studied by Density Functional Theory (DFT) both in gas phase and in water solutions. The decarboxylation without any catalyst is also investigated in order to evaluate the functions of proton and ethylenediamine. The catalyzed mechanisms of oxaloacetic acid proposed by Leussing et al. are confirmed by our calculations. The calculated results show that the dehydration of IM4 is the rate-determining step for the whole catalytic process, agreeing with the conclusions drawn by Thalji et al. And the energy barrier for the decarboxylation of IM6 is only 6.48 kcal/mol, much lower than the reaction without any catalysts or catalyzed with ions.