Complex dielectric permittivity and magnetic permeability of several commercially available nanoferrites have been studied over a broad microwave and millimeter wave frequency range. Nano-sized barium, strontium, copper, zinc, nickel substituted iron oxide powders with different lattice structures are investigated. A transmission-reflection based in-waveguide technique that employs a vector network analyzer was used to determine the scattering parameters for each sample in two microwave bands (18-40 GHz). A free space quasi-optical spectrometer energized by backward wave oscillators was used to acquire the transmittance spectra in the millimeter wave frequency range (30-120 GHz). Relatively broad and sharp ferromagnetic resonance of hexagonal barium ferrite and strontium ferrite are observed in millimeter wave frequency range. The ferromagnetic resonance peak for nano-sized hexagonal ferrite powder material moves to lower frequencies compared to micro-sized and solid hexagonal ferrites. An X-ray diffraction measurement is performed on these hexagonal ferrites to understand the magnetic behavior and the structure.