Magnetite (Fe3O4) hollow nanospheres with an average diameter of 300 nm and an average shell thickness of 40 nm were synthesized by a surfactant-free solvothermal reduction method, and their structure and electromagnetic (EM) properties were investigated. The Fe3O4 hollow nanospheres showed single-crystalline features along the [111] crystal growth direction and a ferrimagnetic behavior at room temperature. The Fe3O4 hollow nanosphere/paraffin composites exhibited a flatter response in the real complex relative permittivity (epsilon') and a lower value of -0.5 in the imaginary complex relative permittivity (epsilon") in comparison with other Fe3O4-based nanomaterials because of the enhanced electrical resistivity. Their imaginary complex relative permeability (mu") displayed a resonance peak at -4 GHz and a negative value up to -0.03 in the 17.2-18 GHz range due to the dissipation of EM energy in the cavity of the hollow nanospheres. Their reflection loss (RL) exceeded -10 dB from 3.1 to 10.1 GHz at a thickness of 2.6-5 mm and attended an optimal value of -43.5 dB at 4 GHz at 5 mm thickness as a result of an effective complementation between the dielectric and magnetic losses.