SiO 2 -Al 2 O 3 supports with different composition were prepared by a precipitation method in order to control the acid property of SiO 2 -Al 2 O 3 supports. NiMo/ SiO 2 -Al 2 O 3 catalysts were then prepared by an impregnation method and sulfided for use in the production of middle distillate through hydrocracking of paraffin wax. It was revealed that MoS 2 phase was dominantly formed in the NiMo/SiO 2 -Al 2 O 3 catalysts, while Ni-Mo-S phase was not detected due to its fine dispersion. Acidity of NiMo/SiO 2 -Al 2 O 3 catalysts measured by NH 3 -TPD experiments showed a volcano-shaped curve with respect to SiO 2 content. Acidity of the catalysts played an important role in determining the catalytic performance in the hydrocracking of paraffin wax. Conversion of paraffin wax was monotonically increased with increasing acidity of the catalyst, while yield for middle distillate showed a volcano-shaped curve with respect to acidity of the catalyst. Thus, optimal acidity of NiMo/SiO 2 -Al 2 O 3 catalysts was required for the maximum production of middle distillate through hydrocracking of paraffin wax.