The effects of a magnesium promoter on the catalytic properties of kieselguhr-supported nickel catalysts were examined through a triglyceride oil hydrogenation reaction. The catalysts were characterized with respect to their BET surface area, pore size distribution, hydrogen chemisorption, temperature-programmed reduction (TPR), and X-ray diffraction (XRD). The reactions were performed at 180℃ and 45 psi. By adding a small amount of magnesium to a kieselguhr-supported nickel catalyst, the BET surface area increases as a result of an increase in microporosity. With the addition of magnesium, the catalysts were more difficult to reduce, but presented a higher metallic area. In the hydrogenation of triglyceride oil, the 4 wt% magnesium-promoted catalyst showed the highest activity, which appeared to be due to an increase of the metallic area as well as maintaining mesoporosity to avoid limited diffusion.