Catalase is an antioxidantenzyme existing in most of the animalcells; it defends them from oxidative damage by catalyzing the rapid decay of hydrogen peroxide in two kinds of reactions. The aim of the present study was to assess the catalase activity in normal subjects and patients whom suffer from diabetes mellitus. Serum catalase activity was assessed by spectrophotometric method in 120 diabetic patients (type II) and 60 healthy subjects. Catalase activity increased in diabetic patients compared to control group. The increase in the serum catalase enzyme is belonged to the oxidative damage of human tissues by increament oxygen free radicals in the body. To correlate our results with the catalase thermodynamically behavior, the thermodynamic constants of activation ÄH, Ea and ÄS, were determined using Arrhenius plot and found to be 11.96 KJ/Mol, 22.44 KJ/male and 2.39 KJ/Mol/K, respectively.