Al2O3-Promoted NiSO4/TiO2 catalysts containing different nickel sulfate and Al2O3 contents were prepared by the impregnation method. The support, Al2O3-added TiO2, was prepared by the coprecipitation method using a mixed aqueous solution of titanium tetrachloride and aluminum nitrate solution, followed by adding an aqueous ammonia solution. The addition of nickel sulfate (or Al2O3) to TiO2 shifted the phase transition of TiO2, from amorphous to anatase, to higher temperatures because of the interaction between nickel sulfate (or Al2O3) and TiO2. 15-NiSO4/5-Al2O3-TiO2 containing 15 wt% NiSO4 and 5 mol% Al2O3, and calcined at 400 ℃, exhibited the maximum catalytic activity for ethylene dimerization. NiSO4 and 5 mol% Al2O3, and calcined at 400 ℃, exhibited the maximum catalytic activity for ethylene dimerization. NiSO4/Al2O3-TiO2 catalysts were very effective for ethylene dimerization, even at room temperature, but Al2O3-TiO2 without NiSO4 did not exhibit any catalytic activity at all. The catalytic activities were correlated with the acidity of the catalysts as measured using the ammonia chemisorption method. The addition of Al2O3 up to 5 mol% gradually enhanced the acidity, surface area, thermal properties, and catalytic activities of the catalysts, as a result of the interaction between Al2O3 and TiO2 and due to consequent formation of Al2O3 up to 5 mol% gradually enhanced the acidity, surface area, thermal properties, and catalytic activities of the catalysts, as a result of the interaction between Al2O3 and TiO2 and due to consequent formation of Al-O-Ti bonds. The higher the frequency of the asymmetric S=O stretching vibration in NiSO4/Al2O3-TiO2 catalyst, the larger both the acidity and catalytic activity toward ethylene dimerization.