A trickle-bed reactor model has been used in this work to simulate the catalytic hydrotreating of oil fractions. The most important reactions (hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and the hydrogenation of aromatics (HDA)) are taken into account in the model. Kinetics of each reaction and reactor model was taken from different sources in the literature. HDS reaction was described by Langmuir-Hinshelwood kinetics. HDN was modeled as a consecutive reaction scheme in which nonbasic compounds are hydrogenated first to basic nitrogen compounds, which undergo further reactions to eliminate the N atom from the molecule. The kinetics of aromatics hydrogenation was represented by a first-order reversible reaction. The reactor model includes correlations for determining mass-transfer coefficients and is based on the two-film theory. The model was tested with experimental information obtained at pilot scale during the hydrotreating of vacuum gas oil. The pilot reactor was operated under isothermal conditions, and the effect of reaction temperature and space velocity on HDS, HDN, and HDA was studied in the ranges of 360-380 degreesC and 2 h(-1), respectively. The predicted results showed good agreement with the experimental information. The model was also applied for simulating the behavior of a NiMo catalyst during commercial nonisothermal operation of an industrial hydrotreating plant.