We report on core-shell iron and iron oxide nanoparticle prepared by plasma synthesis controlled by process gas mixture. The synthesis was carried out using an atmospheric pressure microwave electrodeless plasma jet. We focused on phase composition control and better passivation. We studied the influence of O2, H2 and N2 admixtures to the Ar process gas on the properties of the prepared nanomaterial. Similarly to thermal process, the oxygen admixture preferentially produced Fe2O3 nanoparticles whereas hydrogen admixture resulted in particles with higher metallic content. However, in contrast to thermal process, the addition of nitrogen did not produce iron nitride, but iron/iron-oxide core-shell nanoparticles with enhanced chemical stability.