A series of catalysts, NiSO4/SiO2-Al2O3, for ethylene dimerization were prepared by the impregnation method using aqueous solution of nickel sulfate. Although SiO2-Al2O3 without NiSO4 was inactive as catalyst for ethylene dimerization, the SiO2-Al2O3 with NiSO4 exhibited high catalytic activity even at room temperature. The high catalytic activity of NiSO4/SiO2-Al2O3 was closely correlated with the increase of acidity and acid strength due to the addition of NiSO4. The sample having 15 wt% of NiSO4 and calcined at 500 degreesC for 1.5 h exhibited maxima for catalytic activity and acidity. In view of IR results of CO adsorbed on NiSO4/SiO2-Al2O3, it is concluded that the active sites responsible for ethylene dimerization consist of a low-valent nickel, Ni+, and an acid.