The complex permittivity and permeability of superparamagnetic iron-oxide nanoparticles has been quantified using a circular waveguide assembly with a static magnetic field to align the nanoparticle's magnetization. The high sensitivity of the measurement provides the precise resonant feature of nanoparticles. The complex permeability in the vicinity of ferromagnetic resonance (FMR) is in agreement with the nanoparticle's measured magnetization via conventional magnetometry. A rigorous and self-consistent measure of complex permittivities and permeabilities of nanoparticles is crucial to ascertain accurately the dielectric behaviour as well as the frequency response of nanoparticle magnetization, necessary ingredients when designing and optimizing magnetic nanoparticles for biomedical applications.Magnetic nanoparticles, with their diameters varying from nanometers to hundreds of nanometers, are under extensive investigation currently for biomedical applications, from MRI contrast agents, microwave imaging enhancement to hyperthermia and drug delivery [1][2][3][4]. The principle attraction of magnetic nanoparticles for these applications is that their sizes are significantly smaller than, e.g. target cells, so that magnetic nanoparticles can be in close contact, and their manipulation through their frequency resonant response from external magnetic fields makes it possible to affect or probe cells and tissue. An outstanding challenge to enable such applications has been a thorough understanding of the nanoparticle's magnetization response (i.e. spin dynamics) when superparamagnetic at high frequencies in complex media such as tissue -knowledge revealed by way of the complex permittivity and permeability that has been problematic to measure.In principle, a calculation based on the Debye equation and the Kramers-Krönig relations should predict the static and dynamic magnetic behavior of nanoparticles. [5][6][7] However, values of the required parameters are often an open question as they could be significantly affected by the material, size and measuring device fabrication process.[7] To effectively utilize magnetic nanoparticles for biomedical applications over the frequencies of interest (often in the microwave regime) it is important to accurately characterize not only the magnetic response quantified by the permeability of the nanoparticles, but also the electric response ascertained by the permittivity. [8] Transmission methods could be used for determining the permittivity (ǫ) and permeability (µ) of material over the requisite wide range of frequencies.[8-10] Usually, ǫ and µ are obtained by measuring reflection and transmission coefficients in free space or via waveguide. Calibration is required to adjust the reference plane to sample surfaces using complicated, multi-step processes includ-ing through-reflection-line calibration, which demand expert technique and complex instrumentation.[10] Irrespective of calibration, an accurate measurement of ǫ and µ is very challenging for nanoparticles due to the nano...