The present work investigates the hydrotreating process of a diesel in order to achieve lower sulphur and aromatics content. The entire work was performed in a Hydrodesulphurization (HDS) pilot plant unit located in Chemical Process Engineering Research Institute (CPERI). For the tests, a commercial HDS catalyst (CoMo) was used while the feed was provided by the deep desulphurization unit of a Greek refinery (Motor-Oil refinery). For the determination of diesel aromatics, a method based on the ASTM D-2549-85 was applied. The objective of the work was to investigate the ability of a typical HDS catalyst for aromatics saturation. The effect of the main hydrotreating operating parameters (T, P, WHSV, H 2 /Oil ratio) on sulphur and aromatics removal was also investigated. In general, the data showed that the product density and the aromatic and sulphur content of diesel decreased as the temperature or pressure increased or space velocity decreased. It was concluded that with the present catalyst an aromatics saturation degree of up to 40% could be achieved, giving a diesel product with aromatics content of about 20-25% wt. However, high temperatures (>370°C) were required in order to achieve 500 ppmw sulphur in this feedstock.