NiSO4/gamma-Al2O3 shows very high catalytic activity in alkene oligomerization with good selectivity for dimers. Its acidity plays a very important role in the catalytic behavior. However, it is the acid site with an acid strength of H-0 less than or equal to -3.0 that is related to the oligomerization of alkenes. Comparatively strong acidity is produced through the interaction of sulfate anions and gamma-Al2O3 during calcination. Ni(II) is partially reduced to Ni(I) by reacting with ethene and propene at ambient temperature or evacuation at high temperatures. Dimerization of ethene can be considered as coordination catalysis at active sites of Ni(I), while oligomerization of propene proceeds via dual catalysis due to both coordination catalysis and acid catalysis. But-1-ene is catalytically oligomerized via mainly acid catalysis. Thus, it can be concluded that as the number of carbon atoms of the alkenes increases, the mechanism of oligomerization of the alkene over the catalyst shifts from coordination catalysis to acid catalysis.