The deactivation of a Ni-Mo-W/Al_2O_3 catalyst during ultra-low-sulfur diesel production was investigated in a pilot plant. The reasons of catalyst deactivation were analyzed by the methods of elemental analysis, BET and TG-MS. The results showed that the catalyst deactivation rate was notable at the beginning of run, and then gradually reached a steady state after 448 h. In the initial period the catalyst deactivation may mainly be caused by the formation of the carbon deposits. The carbon deposits blocked the catalyst pores and the accessibility of active center decreased. The TG-MS analysis identified three types of carbon species deposited on the catalysts, viz.: the low temperature carbon deposit with high H/C atomic ratio, the medium temperature carbon deposit, and the high temperature carbon with low H/C atomic ratio. The amount of high temperature carbon deposits on the catalyst determined the overall activity and, therefore the high temperature carbon was a major contributor to the deactivation of Ni-Mo-W catalyst. The deactivation of a Ni-Mo-W / Al 2 O 3 catalyst during ultra-low-sulfur diesel production was investigated in a pilot plant. The reasons of catalyst deactivation were analyzed by the methods of elemental analysis, BET and TG-MS. that the catalyst deactivation rate was notable at the beginning of run, and then gradually reached a steady state after 448 h. In the initial period the catalyst deactivation may mainly be caused by the formation of the carbon deposits. and the accessibility of active center decreased. The TG-MS analysis identified three types of carbon species deposited on the catalysts, viz .: the low temperature carbon deposit with high H / C atomic ratio, the medium temperature carbon deposit, and the high temperature carbon with low H / C atomic ratio. The amount of high temperature carbon deposits on the catalyst determines the overall activity and, therefore the high temperature carbon was a major contributor t o the deactivation of Ni-Mo-W catalyst.