We report fabrication, structural and magnetic properties of cobalt nanocrystals. They are synthesized in colloidal assemblies. The coated cobalt particles are air stable. Due to the narrow size distribution of these 8 nm particles, they self-assembled after deposition on a flat substrate. The magnetic response depends on the organization. When they are dispersed in a solution, the saturation is not reached at 2T even at 3 K, they are still a superparamagnetic part in the signal. When particles are organized in 2D superlattices, saturation is reached at lower applied field, compared to isolated particles. This is attributed to an increase of dipolar interaction. However, the hysteresis loop markedly differs with the sample orientation in the applied field. This confirms the dipolar magnetic interactions between cobalt nanoparticles.