NiO-ZrO₂/SO₄^-2 catalysts were prepared by coprecipitation from the solution of nickel chloride-zirconium oxychloride mixture followed by treating with H₂SO₄. NiO-ZrO₂ alone without SO₄^-2 was inactive as a catalyst for ethylene dimerization, but NiO-ZrO₂/SO₄^-2 was found to be very active even at room temperature. The high catalytic activity of NiO-ZrO₂/SO₄^-2 was closely correlated with the increase of acid strength by the inductive effect of sulfate ion adsorbed on NiO-ZrO₂. The decrease of catalytic activity above 450℃ of evacuation temperature was explained in terms of the structural change of catalyst from amorphous phase to crystalline, and the sintering.