Two types of layer nickel silicate, nickel montmorillonite and nickel antigorite were synthesized by co-precipitation from an acidic solution of a nickel salt-sodium silicate mixture followed by hydrothermal treatment at 275℃ and 780psi. The structures of synthesized nickel silicate were examined by X-ray diffraction patterns and in-frared absorption spectra. The nickel silicates thus synthesized were activated for ethylene dimerization by evacuation at elevated temperatures, giving two optimum temperatures, 100℃ and 600℃. The acid site responsible for the catalyt-ic activity is Bronsted acid at low evacuation temperature of 100℃, while Lewis acid at high evacuation temperature of 600℃. The crystallinity of the nickel silicates was improved gradually with the hydrothermal treatment, while the catalytic activities and surface area decreased with that.