This paper describes a model to predict the behavior of trickle-bed reactors used for catalytic hydrotreating of oil fractions with cocurrent and countercurrent operation modes. A dynamic plug-flow heterogeneous one-dimensional model, which has been previously validated with experimental data obtained in an isothermal pilot plant reactor with cocurrent operation, was employed to compare both modes of operation. The reactor model considers the main reactions present in the hydrotreating process: hydrodesulfurization, hydrodenitrogenation, and hydrodearomatization. Simulations were performed for both pilot and commercial trickle-bed reactors, and the results are discussed in terms of variations with time and axial position of partial pressure, temperature and concentrations in liquid phase. A superior performance of countercurrent operation mode was found over cocurrent mode. It was recognized that countercurrent mode can have great potential to be used for deep hydrodesulfurization of oil fractions since it minimizes the inhibiting effect of some products (e.g. H2S) in reactor zones where these species tend to concentrate in cocurrent operation, i.e. at the bottom of the catalytic bed. (C) 2007 Elsevier B.V. All rights reserved.