A series of catalysts, NiO-ZrO2/SO42-, for ethylene dimerization were prepared by coprecipitation from a solution of a nickel chloride-zirconium oxychloride mixture followed by modifying with H2SO4. On the basis of the results obtained from X-ray diffraction and differential thermal analysis, the addition of nickel oxide to ZrO2 or modification with H2SO4 shifted the transition of ZrO2 from amorphous to a tetragonal phase at higher temperatures due to the interaction between nickel oxide (or sulfate ion) and ZrO2. Infrared spectra of the catalyst modified with H,SO, showed bidentate sulfate ion coordinated to Zr4+ Or Ni2+. NiO-ZrO2 Without sulfate ion was inactive for the ethylene dimerization, but NiO-ZrO2/SO42- was found to be very active even at room temperature. The high catalytic activity of NiO-ZrO2/SO42- was closely correlated with the increase of acid strength by the inductive effect of sulfate ion. The decrease of catalytic activity above 450 degrees C of evacuation temperature was due to the structural change from amorphous phase to crystalline and the sintering followed by the decrease of surface area.