The present invention relates to a thermo catalytic process to produce diesel oil from vegetable oils, in refineries which have two or more Catalytic Cracking (FCC) reactors. At least one reactor processes heavy petroleum or residue in conventional operation conditions while at least one reactor processes vegetable 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 may improve efficiency for the obtainment of highly pure products and may not yield glycerin, one by-product of the transesterification process. The diesel oil produced by said process may have superior qualities and/or a cetane number higher than 40. Once cracking conditions occur at lower temperatures, it may form a less oxidized product, which is consequently purer than those obtained by existent technology.