Obtaining stable metal nanoparticles is of high interest for various applications such as catalysis, batteries, supercapacitors and electro-magnetic devices. Cobalt/multi-walled carbon nanotubes (MWCNT) hybrids with an original set of magnetic and electric properties were formed by casting Co nanoparticles (3 – 5 nm) of high aspect ratios within the internal space of MWCNTs. The Co particles localization and size were analyzed by transmission electron microscopy and synchrotron x-ray diffraction. The magnetism of the cobalt nanoparticles was probed by 59Co internal field nuclear magnetic resonance (IF NMR) and their electrical behavior by dielectric spectroscopy. The majority of Co particles were fully metallic. They resisted sintering up to 550 °C. Below 7.5 wt%, the Co was exclusively embedded inside the MWCNT. At higher loading, they coexisted with larger Co outside particles. While nanometer size particles are normally superparamagnetic at room temperature, the confinement of Co within MWCNTs resulted in a ferromagnetism revealed by 59Co IF NMR. This spectroscopy provided original information about the structure, size, and shape anisotropy of the nanoparticles. Finally, the MWCNT modification by Co metal nanoparticles improved the electrical conductivity of polyethylene based composite thus extending the useful frequency band of Co/MWCNT/PE composites for applications requiring light-weight conduction or energy absorption.