NOVELTY - An alkylphenol-based antioxidant additive composition for stabilizing biodiesel fuel comprises (mass-%) 2,6-di-tert-butyl-4-methyleyelohexanone (0.2-0.3); 2,6-di-tert-butylphenol (0.7-6.0); 2-sec-6-tert-butyl-p-cresol (1.5-5.0); 4,6-di-tert-butyl-o-cresol (3.0-8.0); 2,4-di-tert-butylphenol (0.3-0.5); 2,4-di-tert-butyl-6-dimethylaminomethylphenol (2.0-5.0); 4,4-methylene-bis(2,6-di-tert-butylphenol) (0.1-0.3); and 2,6-di-tert-butyl-4-methylphenol (to the balance).    USE - The alkylphenol-based antioxidant additive composition is useful for stabilizing biodiesel fuel; and for improving storage stability of biodiesel fuel (all claimed). Generally, it can be used in petrochemical industry.    ADVANTAGE - By using the alkylphenol-based antioxidant additive composition, the technical result comprises a considerable improvement of the storage stability of biodiesel fuel with a reduced amount of the antioxidant additive. The antioxidant additive inhibits precipitation during the shelf life of biodiesel fuel for a longer period of time.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are:    (1) a solution of the alkylphenol-based antioxidant additive composition for addition to biodiesel fuel comprising alkylphenol-based antioxidant additive composition with a concentration of from 6 to 48 mass-% in the biodiesel fuel; and    (2) a method for improving storage stability of biodiesel fuel comprising addition of an alkylphenol-based antioxidant additive, where the alkylphenol-based additive is the composition above, where a solution is provided containing from 6 to 48 mass-% of the composition dissolved in biodiesel fuel, and the solution is added to the biodiesel fuel to reach a concentration of the composition of from 0.002 to 1.6 mass-% based on the entire biodiesel fuel solution; or a method for improving storage stability of biodiesel fuel comprising addition of an alkylphenol-based antioxidant additive, where the antioxidant additive is added as a solution of the antioxidant additive composition to reach a concentration of the composition of from 0.002 to 1.6 mass-% based on the entire biodiesel fuel solution.