Spherical nickel nanoparticles with different sizes and different structures were prepared starting from nickel oxalate and using hydrazine hydrate as a reductant in the presence of citric acid, cetyltrimethylammonium bromide, Tween 40, and D-sorbitol as organic modifiers. The size and structure of the resultant nickel nanoparticles were affected by using different organic modifiers. All Of the nickel nanoparticles showed higher catalytic activity and selectivity than the conventional raney Ni catalyst in the hydrogenation of p-nitrophenol to p-aminophenol. Although the catalytic activities of the resultant nickel nanoparticles generally increased with decreasing particle size, the structure of nickel nanoparticles also had all important impact on the catalytic activity.