An experimental study for comparing the behavior of a small-reactor in the hydrodesulfurization of straight-run gas oil with two modes of operation, upflow and downflow, is presented. The reactor has 525 min length and 14.3 min internal diameter. Experiments were conducted at commercial HDS operating conditions (5.3 MPa total pressure, 356.2 mL/mL hydrogen-to-oil ratio, reaction temperature of 613-653 K, and LHSV of 1.0-2.5 h(-1)). All tests were carried out over a commercial NiMo/gamma -Al2O3 catalyst. The effect of different catalyst-to-diluent ratios on the sulfur removal was also studied. The results of SRGO hydrodesulfurization showed that upflow with a 50/50 vol % catalyst-to-diluent ratio is the best system to avoid external gradients and other effects such as poor wetting of catalyst, axial dispersion, wall effects, and flow maldistribution. Apparent activation energies were determined for upflow and downflow modes of operation with and without diluent.