A series of Ni-W/Al2O3-SiO2 catalysts with different Al2O3/SiO2 ratios were prepared and loaded in a commercial HDS unit for a year. The catalytic properties of the freshly sulfided and spent catalysts were investigated by performing structural analyses and model test reactions, The aggregation of the WS2 slabs in the lateral direction, which was observed by EXAFS and TEM, was a major cause of the ’catalyst deactivation. The addition of SiO2 into the catalyst support was found to suppress the structural changes of the catalyst.