Dispersed nanostructures of nickel (Ni) have been synthesized by thermal decomposition of nickel-oleate in the presence of 1-octadecene with controlled synthesis temperature. The evolution of face-centered-cubic (fcc) phase with increasing synthesis temperature from 320 to 365°C leads to structural phase transformation of nickel nanostructures from hexagonal-close-packed (hcp) to fcc through mixed phases. The saturation magnetization (MS) of pure fcc and hcp is ∼37 and ∼0.67emu/g, respectively. The quenched M S of hcp Ni nanostructure compared to that of the fcc Ni indicates the presence of frustrated or canted spins in it. As the fcc phase fraction increases the MS increases, but the observed MS is significantly larger than the theoretical MS calculated by considering the contribution solely from the pure hcp and fcc Ni. This enhanced MS indicates the presence of exchange coupling between the phases and nanoparticles. © 2014 AIP Publishing LLC.