Abstract: The present invention comprises a thermo catalytic process to produce diesel oil from vegetal oils, in refineries which have two or more Catalytic Cracking (FCC) reactors. At least one reactor processes heavy gasoleum or residue in conventional operation conditions while at least one reactor processes vegetal oils in proper operation conditions to produce diesel oil. This process employs the same catalyst employed in the FCC process, which processes conventional feedstocks simultaneously. This process transforms high heat content raw materials into fuel hydrocarbons. It shows excellent efficiency for the obtention of highly pure products and do not yield glycerin, one by-product of the transesterification process. The diesel oil produced by said process presents quality superior and cetane number higher than 40. Once cracking reactions occur at lower temperatures, it forms less oxidized product, which is consequently purer than those obtained by existent technology are.