A comparative study of structure, stability and magnetic properties of Ni nanoparticles (NPs) synthesized by NaBH4 reduction method (SBR) and Polyol process (PP) has been carried out. Temperature and field dependence magnetization measurements suggest that 600 ºC air annealed for 1 hour samples exhibit weak and strong magnetic properties for samples prepared by PP and SBR methods respectively. From the structural and magnetic measurements we conclude that Ni particles prepared by SBR and annealed at 600 ºC air develops in to core-shell structure NPs while PP NPs under similar conditions show heterogeneous magnetic (Ni/NiO) behavior. © 2020 American Institute of Physics Inc.. All rights reserved.