NOVELTY - Hydrotreating biomass oil diluted in a refinery stream of petroleum hydrocarbons, using of sulfide catalytic beds, involves diluting volume of biomass oil and/or fatty acid in refinery stream of petroleum hydrocarbons with distillation of 140-500 degrees C to obtain feedstock containing the biomass oil (1-75 wt.%); mixing hydrogen stream with the feedstock; injecting upstream of second catalytic bed a dilution stream containing hydrogen; and separating the gaseous products, recovering stream of liquid hydrocarbons containing dissolved gases, and purifying the stream to remove dissolved gases.    USE - For hydrotreating biomass oil diluted in a refinery stream of petroleum hydrocarbons (claimed).    ADVANTAGE - The process provides a first reaction zone for hydroconversion of the triglycerides and/or fatty acids into corresponding n-paraffins 12-18C. The process provides conversion and selectivity for the production of fuels using HDT petroleum refinery units, combining the control of the triglycerides hydroconversion reactions with that of the hydrogenation reactions of the unsaturated hydrocarbons and the removal of heteroatomic contaminants from the refinery petroleum hydrocarbon stream; and a solution to the problem of generation of gases from the hydroconversion of triglycerides into n-paraffins, in co-processing of a biomass oil diluted in a usual refinery stream of petroleum hydrocarbons in a hydrotreating-HDT unit in petroleum refining.    DETAILED DESCRIPTION - Hydrotreating biomass oil diluted in a refinery stream of petroleum hydrocarbons, using of sulfide catalytic beds consisting of (i) a first monometallic catalytic bed containing only one VIB metal oxide and (ii) at least one second bimetallic catalytic bed for hydrotreating the refinery stream, involves (a) diluting a volume of biomass oil containing more than 70 wt.% of at least one triglyceride and/or fatty acid in a refinery stream of petroleum hydrocarbons with a distillation of 140-500 degrees C to obtain a feedstock containing the biomass oil (1-75 wt.%); (b) mixing a hydrogen stream with the feedstock to obtain a first reaction mixture, which is heated and injected in the first catalytic bed at 220-370 degrees C, causing it to react under a pressure of 3-15 MPa; (c) injecting upstream of a second catalytic bed a dilution stream containing hydrogen to obtain a second reaction mixture, which is hydrotreated under conditions for hydrotreating petroleum hydrocarbons, such that gaseous products and a stream of liquid hydrocarbons are obtained; and (d) separating the gaseous products, recovering a stream of liquid hydrocarbons containing dissolved gases, and purifying the stream to remove dissolved gases to obtain a hydrotreated liquid product.