The present invention relates to a process for manufacturing stable and monodisperse biocompatible bismuth metallic (0) nanoparticles, in aqueous solution, using a water-soluble bismuth precursor, a water-soluble reducing agent and a coating agent. The process can be carried out in continuous flow. The invention also relates to corresponding stable biocompatible bismuth metallic (0) nanoparticles having hydrophilic, hydrophobic or fluorophilic properties and uses thereof, especially medical applications thereof such as theranostic applications.