A process for preparing a diesel hydrocarbon fraction which comprises the following phases: (a) reacting at least a renewable feedstock comprising glycerides with water at a temperature within the range of 150-400°C and a pressure within the range of 1-20 MPa, forming at least a biphasic mixture consisting of an aqueous phase containing glycerine and an organic phase comprising a mixture of water insoluble fatty acids; (b) separating said glycerine from said mixture of fatty acids; (c) subjecting said mixture of fatty acids to catalytic hydrotreatment in the presence of hydrogen, obtaining a reaction product comprising: (i) at least one gaseous effluent comprising hydrogen, water vapour and CO 2 and possibly CO, (ii) at least one diesel hydrocarbon fraction comprising branched paraffins in a prevalent quantity with respect to the linear paraffins, said catalytic hydrotreatment being carried out at a temperature within the range of 240-450°C and a pressure within the range of 1-20 MPa in the presence of a catalytic composition comprising at least one metal of group VIIIB dispersed on at least one acid carrier for catalyzing a hydroisomerization reaction of linear paraffins to branched paraffins, wherein said at least one acid carrier is selected from: zeolite in acid form, silico-alumina, ASA, SAPO, MSA, MSA-P or mixtures thereof; (d) separating said diesel hydrocarbon fraction from said reaction product. (a) reacting at least a renewable feedstock comprising glycerides with water at a temperature within the range of 150-400°C and a pressure within the range of 1-20 MPa, forming at least a biphasic mixture consisting of an aqueous phase containing glycerine and an organic phase comprising a mixture of water insoluble fatty acids; (b) separating said glycerine from said mixture of fatty acids; (c) subjecting said mixture of fatty acids to catalytic hydrotreatment in the presence of hydrogen, obtaining a reaction product comprising: (i) at least one gaseous effluent comprising hydrogen, water vapour and CO 2 and possibly CO, (ii) at least one diesel hydrocarbon fraction comprising branched paraffins in a prevalent quantity with respect to the linear paraffins, said catalytic hydrotreatment being carried out at a temperature within the range of 240-450°C and a pressure within the range of 1-20 MPa in the presence of a catalytic composition comprising at least one metal of group VIIIB dispersed on at least one acid carrier for catalyzing a hydroisomerization reaction of linear paraffins to branched paraffins, wherein said at least one acid carrier is selected from: zeolite in acid form, silico-alumina, ASA, SAPO, MSA, MSA-P or mixtures thereof; (i) at least one gaseous effluent comprising hydrogen, water vapour and CO 2 and possibly CO, (ii) at least one diesel hydrocarbon fraction comprising branched paraffins in a prevalent quantity with respect to the linear paraffins, said catalytic hydrotreatment being carried out at a temperature within the range of 240-450°C and a pressure within the range of 1-20 MPa in the presence of a catalytic composition comprising at least one metal of group VIIIB dispersed on at least one acid carrier for catalyzing a hydroisomerization reaction of linear paraffins to branched paraffins, wherein said at least one acid carrier is selected from: zeolite in acid form, silico-alumina, ASA, SAPO, MSA, MSA-P or mixtures thereof; (d) separating said diesel hydrocarbon fraction from said reaction product.