Cetane improver for diesel fuels, characterized in that it comprises at least one ester of stearic acid, selected from the esters of linear or branched monoalcohols containing 1 to 6 carbon atoms: a) added in the pure state to a mixture of at least one vegetable or animal oil ester in crude or partially hydrogenated form b) or else included in a mixture of esters of one or more vegetable and/or animal oils, in crude or partially hydrogenated form, the mixtures of saturated or unsaturated esters being such that the ratio by mass of the amount of stearic ester(s) to the total sum of the amounts of unsaturated esters present in the vegetable or animal oil esters varies from 1% to 12%, and the unsaturated fatty acid esters being esters of mono or polyunsaturated acids containing at least 18 carbon atoms with monoalcohols.