A series of dealuminated NiDY zeolites was prepared by the reaction of NH4NaY with SiCl4 at elevated temperatures. followed by exchanging with NiCl2 solution. The acid strength and thermal stability of dealuminated zeolite samples increased with the dealumination. Upon dealumination up to Si/Al ratios of 6.0, the increases of catalytic activities for ethylene dimerization were correlated to the OH stretching band shifts in infrared spectra and to the increase of acid strength for the dealuminated NiDY samples.