The combination of ferrite and carbon materials with core-shell nanostructures has attracted tremendous interest in microwave absorption field. Herein, Fe/Fe3C-C core-shell nanoparticles with diameters of 10-70 nm were synthesized by one-step metal-organic chemical vapor deposition. The nanocores of the nanoparticles can be tailored from Fe3C, to Fe3C/Fe, and then Fe by simply changing the reaction temperature. The Fe/Fe3C-C core-shell nanoparticles exhibited an optimal reflection loss (RL) value of -58.5 dB and effective absorption bandwidth (frequency range for RL < - 10 dB) of 6.2 GHz at a low thickness of 2.0 mm. The microwave absorption performances of the as-synthesized Fe/Fe3C-C core-shell composites could be easily tuned by the phase composition of nanocores. The achieved results demonstrated that the as-synthesized Fe/Fe3C-C core-shell nanoparticles have considerable potential as an effective microwave absorber with tunable absorption performance.