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Extracts of Cotoneaster medicus, Glycyrrhiza glabra, as endemic plants of Iran, along with mixtureof them were investigated for their antioxidant activities using 2, 2- diphenyl-1-picrylhidrazyl (DPPH) reagent.UV-Vis spectrophotometry method was used to evaluate the ability of Cotoneaster and Glycyrrhiza glabraantioxidant to scavenge DPPH radical. The kinetic parameters such as rate constant and activation energy inexperimental conditions were calculated. The rate constants of the H atom abstraction by DPPH (k1), in thepresence of C. medicus and G. glabra antioxidant were obtained under pseudo-first-order conditions atdifferent temperatures. The order in DPPH radical-scavenging was: mixture of C. medicus and G. glabra > C.medicus > G. glabra plants. The numerical values of activation energy were found to be 45.84 kJ.mol-1for G.glabra and 62.02kJ.mol-1 for C. medicus.