GdNi_5 nanoparticles and GdNi_5/Gd_2O3nanocapsules(with GdNi_5 core and Gd_2O_3 shell) were prepared by arcdischarge technique under different hydrogen partial pressure.The GdNi_5 nanoparticles show irregular spherical shape and have a size distribution of 10—50 nm with an average diameter of 15 nm.In comparison,the GdNi_5/Gd_2O_3 nanocapsules present spherical morphology and show a size distribution of 10—100 nm with an average diameter of 60 nm.Under a magnetic field change of 50 kOe,the maximum magnetic entropy change of GdNi_5 nanoparticles is 13.5 J/(kg K) at 5 K,while the corresponding value of the GdNi_5/Gd_2O_3nanocapsuels is only 5.7 J/(kg K) at 31 K.The origin of the large magnetic entropy change of GdNi_5nanoparticles is ascribed to its high atomic moments and small anisotropy energy barrier induced by its small particle size. GdNi_5 nanoparticles and GdNi_5 / Gd_2O_3 nanocapsules (with GdNi_5 core and Gd_2O_3 shell) were prepared by arcdischarge technique under different hydrogen partial pressure. GdNi_5 nanoparticles show irregular spherical shape and have a size distribution of 10-50 nm with an average diameter of 15 nm. In comparison, the GdNi_5 / Gd_2O_3 nanocapsules have a spherical morphology and show a size distribution of 10-100 nm with an average diameter of 60 nm. Bunker a magnetic field change of 50 kOe, the maximum magnetic entropy change of GdNi_5 nanoparticles is 13.5 J / (kg K) at 5 K, while the corresponding value of the GdNi_5 / Gd_2O_3 nanocapsules is only 5.7 J / (kg K) at 31 K. The origin of the large magnetic entropy change of GdNi_5nanoparticles is ascribed to its high atomic moments and small anisotropy energy barrier induced by its small particle size.