Reactive oxygen species (ROS) are small, highly reactive, oxygen containing molecules that are naturally generated in small amounts during the body’s metabolic reactions and can react with and damage complex cellular molecules such as fats, proteins, and DNA. Superoxide dismutase (SOD) represents an essential defense system against oxygen-derived free radicals, specifically superoxide (O2•-). Superoxide can initiate a series of free radical reactions that yields other oxygen radicals, which together are thought to act as inflammatory mediators and induce cellular damage. The presence of natural superoxide dismutase (SOD), Catalase (CAT), and Glutathione Peroxidase (GPx) is to perform as defense systems against the cells damage which caused by superoxide compounds. These various enzymes allow to founding a widely area of treatment of many diseases. In our thesis, a series of ten Schiff-bases manganese (III) complexes which is previously prepared have been designed and build as SOD and CAT mimetics. The complexes along with the known Mn-Salen [(C1: a known drug used as standard against superoxide anion (O2 -) (SOD-mimetic activity) and hydrogen peroxide (H2O2) (CAT-mimetic activity)], have been characterized using the Gaussian 03 program (2003). The results show the following: 1. The manganese (III) ion in these complexes is surrounded by two nitrogen atoms of the Schiff-base, and two oxygen atoms and metal ion above the basal plane toward the anion (Cl-). 2. The behavior of building complexes is different in the oxidationreduction reactions. 3. Some complexes which have mixed ligands (heteroligands), show to have better activities than that of the same ligands (homoligands). 4. Some complexes get high mimetic enzyme activity in one reaction (SOD-mimetic or CAT-mimetic activities), but low mimetic enzyme activity in the other reactions. 5. The activities of both C2 and C7 complexes are higher than that of C1 complex in both reaction types (SOD-mimetic and CAT-mimetic activities). 6. The activities of (C4, C9, C10) complexes are higher mimetic enzyme than that of C1 complex as superoxide dismutase. 7. The activities of (C3, C5, C6, C8, C11) complexes are higher mimetic enzyme than that of C1 complex as Catalase. 8. The calculation of geometrical shapes of building complexes were achieved by calculate the bond frequencies and force constant, the results indicate that all the building complexes have square pyramidal shape.