Fabrication, structural, magnetic and local electronic properties of cobalt metallic nanocrystals are reported. Nanosized cobalt particles are synthesized in colloidal assemblies. After surface passivation and extraction of the nanocrystals from the micellar media, cobalt nanoparticles with a narrow size distribution are obtained. The particles are then stable under air. Deposited on a surface, the nanoparticles form a 2D hexagonal network spontaneously. The magnetic properties are strongly dependent on the self-organization of cobalt nanocrystals. In a 2D self-assemblies, collective properties originated from the dipolar magnetic interaction between adjacent particles are observed. Isolated particles are studied by scanning tunneling microscopy at room temperature under air. The spectroscopy experiments performed at 77 K show well-resolved Coulomb gap and Coulomb staircase features.