Method of preparation of antioxidant composition on the basis of diphenylamine by catalytic alkylation of diphenylamine by an threefold excess of diisobutylene with regard to diphenylamine in the presence of an alkylation catalyst t in an amount of 5 to 30% by weight, as referred to diphenylamine, at a temperature of 140 to 160° C. Water contained in the catalyst, is removed and subsequently diphenylamine is left to complete the reaction with diisobutylene and after the reaction is finished, the reaction mixture is separated from the catalyst, unreacted diisobutylene is removed from it and the reaction product is obtained.