CoMoB/Al2O3 catalysts with up to 2 wt.% of B have been synthesized by two different methods an introduction of boron into gamma-Al2O3 support during the kneading of the paste or by the impregnating of gamma-Al2O3 extrudates by the solution of Mo and Co compounds, citric and boric acids. It is shown that the activity in HDS and HDN of diesel fuel increases with increasing boron content in catalysts. The growth of the activity is associated with the increase in CoMoS phase content, Co dispersity, an average slab length of sulfide active component particles and an average number of slabs per 1000 nm(2). The positive effect of boron is much more pronounced for catalysts prepared by the introduction of boron to the solution, since boron is located preferentially on the surface of alumina particles.