The catalytic properties of trimetallic NiMoWS catalysts supported on alumina are studied in the process of straight-run diesel fraction hydrotreating. It is shown that the nature of the oxide precursor of the mixed NiMoWS active phase strongly affects catalyst activity in the hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrogenation (HYD) of polycyclic aromatic hydrocarbons (PAH). The NiMoWS/Al2O3 catalyst synthesized from the mixed H4SiMo3W9O40 heteropoly acid is more efficient than the sample prepared from a mixture of individual H4SiMo12O40 and H4SiW12O40 heteropoly acids in both HDS and HDN transformations at the same metal content. The highest activity in PAH hydrogenation is exhibited by the NiMoWS/Al2O3 catalyst prepared using H4SiMo3W9O40.