Herein, I present a computational study approach to gain insight of the various chemical and physical properties of peroxidases and haloperoxidases activity in enzymes and biomimetic model complexes. Heme enzymes are one of versatile enzyme existing in nature. In Chapter 3 and Chapter 4, we did a detailed study on two types of heme enzymes namely Chloroperoxidase and Cytochrome P450, respectively. Chloroperoxidase is a unique enzyme that are able to neutralize hydrogen peroxide into water and at same time convert halides into hypohalide. While, Cytochrome P450 utilizes O2 and NO on the active site of the enzyme and converts L-tryptophan to 4-nitrotryptophan. Even, both enzymes have similar structure of heme arrangement, however, the arrangement of amino acid residue in their active site is a key factor to determine their type of reaction and capability to react with various substrate. Thus, the understanding of their respective processes by environmentally utilize oxidants might lead for future applications in various fields. The attempt to mimic the structural features of the coordination environment of the natural enzyme always give good result for us to remodel desired reaction. Hence, in Chapter 5, Chapter 6 and Chapter 7 we did a detailed study on biomimetic complexes containing vanadium and iron. Overall, vanadium-oxo form is sluggish oxidants and reacts slowly with their respective substrate. However, the conversion of vanadium-oxo to vanadium-peroxo intermediate by using any peroxide is an important step to make it as better oxidant. In Chapter 7, we did collaboration work with the experimentalist group to demonstrate the incorporation of a dialkylamine group into the second coordination sphere of a FeII complex allows the formation of FeIV-oxo intermediate which is unusual phenomena. In this work, we are able to prevent the formation of FeIII species and hydroxyl radicals where this species prone to engage in uncontrolled radical chemistry. In the biomimetic model complexes study, several factors had been studied that give a significant effect to the rate reaction such as ligand architecture, type of equatorial ligand and second-coordination effect. In Chapter 8 and Chapter 9, we did work to identify how the metal substituent affects the chemical reactivity of biomimetic complexes. We did replaced iron by ruthenium and manganese, and performed a detailed computational study for their respective systems and observe the reactivity of the system. Then we did thermochemical cycles and valence bond patterns analysis to explain the differences in chemical properties and identify how the metal substituent affect the chemical reactivity of complexes with substrates.