To stabilize omeprazole(OMP), ethylenediamine(ED) complex of omeprazole(OMPED) was prepared by reaction between OMP and ED in methanol, and the complex formation was confirmed by the instrumental analysis, i.e., IR, DSC, EA, NMR, MS and XRD. The rates of decomposition of OMP and OMPED in aqueous solution and the shelf lives at standard temperature were measured by accelerated stability analysis. The results are summarized as follows; The mole ratio of OMP and ED in OMPED complex is 1:1, the energy of formation within OMPED might be combined between polar imidazole group of OMP with induced a dipole amine group in the readily polarizable ED molecule. At standard temperature the degradation rate constant of OMP in aqueous solution is $2.540{ imes}10^{-2};hr^{-1}$ and the shelf life is 4.15 hrs, and in the case of OMPED the degradation rate constant is $7.986{ imes}10^{-4};hr^{-1}$ and the shelf life is 131.96 hrs. So, the OMPED has about 31 times longer shelf life than OMP. The activation energy of OMP and OMPED are 5.23 and 18.55 kcal $mole^{-1}$ respectively. The stability of OMP is dependent chiefly on pH in the solutions and it decomposes readily in acidic medium by hydrogen ion catalized reaction but becomes stable beyond pH 9.0. In case of the ED-complex, OMPED is stable in neutral as well as in dilute acidic solutions even in pH 6, OMPED is very stable to light(UV), that is, the rate constant and shelf life of OMP are $k=1.0188{ imes}10^{-2};day^{-1}$, $T_{90%}=4.5 ;days$, on the other hand, the those of OMPED are $k=7.138{ imes}10^{-4};day^{-1}$, $T_{90%}=64.1;days$, respectively. From the above results, it is thought that new dosage forms could be developed by using the OMPED as a potential OMP complex.