A hydrotreating method uses two catalyst beds with the introduction, on the last catalyst bed, of oils of animal or vegetable origin for co-treating a mixture made up of oils of vegetable or animal origin and of petroleum cuts (gas oil cuts (GO) and middle distillates) in order to produce gas oil effluents meeting specifications with an improved cetane number. The first catalyst bed is dedicated to only the deep desulfurization reactions (HDS1) of a petroleum type feed. The effluents of the first catalyst bed having an effluent sulfur content below or equal to 50 mg/kg are separated into two streams. The first stream, which is predominant, is sent to the gas oil pool. The second stream is mixed with oils of vegetable or animal origin. The resultant oil-petroleum cut mixture is then subjected to a milder hydrotreatment (HDT2). The effluents obtained at the outlet of the second catalyst bed can optionally be mixed with the predominant stream from the first bed. The process economy, the tolerance to the specifications relative to oils of animal or vegetable origin and the quality of the products obtained are thus greatly improved.