In order to expand diesel fuel processing capacity an option to co-hydrotreat straight run gas oil blended with fraction containing FCC naphtha and light cycle oil was investigated. Industrial test run was performed under pressure of 40 bar in the catalytic reactor with two layers of conventional Co-Mo/gamma-Al2O3 catalyst, by increasing volume content of FCC naphtha and light cycle oil fraction in the reactor inlet to 20%vol. Reactor inlet temperature was also increased during the test run from 327 to 334 degrees C. Liquid hourly space velocities between 1.05 and 1.32 h(-1) have been used with H-2/oil ratios of 908-1135 Nm(3) hydrogen/m(3) oil. Sulphur inlet concentrations of 6500-8200 ppm were reduced to 36-72 ppm in the hydrotreated oil. GC-MS analysis of the sulphur compound's sub-families has shown that most of the sulphur was present in more reactive classes like alkyl substituted benzothiophenes. High conversion of sulphur was achieved by a combination of the following process parameters: high reaction temperature, low space velocity, presence of FCC naphtha in the feed and distribution of inlet sulphur dominantly concentrated in more reactive benzothiophenes. The activation energy of 33.2 kcal/mol was determined for this type of feedstock using power law kinetic expression. (C) 2012 Elsevier B.V. All rights reserved.