In order to obtain more data on the hydrocracking catalysts, an experiment has been carried out to study the hydrocracking of vacuum distillate into middle distillate with operating conditions: temperature: From 390 to 420°C: LHSV: from 0.45 to 0.90; presRure: 125 kg/cm and HHC ratio: 1000 It/lt by using Two Ni-Mo/Al 03 SIO, catalysts with various particle stzes using a Catatest unit operated in a continuous sys- tem.Gas product sample was analyzed by using a gas-liquid chromatography, The liquid product were fractionated and bottom products were dewaxed by solvent dewaxing.Based on the experimental data show that :By increasing the operating temperature, LHSV, and decreasing of the particle size of the Ni-Mo/Al0Si0, cutalyst, the conversion rate, the quality of the middle distillate, and the lubricant base stocks products in - crease, but the selectivity of these products will decrease.At operating conditions T= 400°C, P-125 kg/cm; LHSV = 0.45; and H/HC ratio : 1000 It/lt, the conversion rate, smoke point of kerosene, diesel index of gas oll, viscosity index of lubricant base stock decrease from: 54.08 to 50.08% by weight; from 22 to 16 mm, from 62.0 to 57.7 and from 133 to 118 for the low particle size and high particle size of the Ni-Mo/Al 0rSiO, catalysts respectively. The apparent activation energy of vacuum distillate is obtained 28,717 and 23,720 kcal/mol for the low par ticle size and the high particle size of the Ni-Mo/Al-0SIO2 catalyst respectively.