In this study, FeB sub(x)Fe sub(2-x)O sub(4) nanoparticles (NPs) were synthesized by the polyol method. The M-H hysteresis curves exhibit superparamagnetic characteristics that are both coercivity and remanent magnetization values are negligible. The particle size dependent Langevin function was applied to calculate the magnetic particle dimensions around 9 nm. The measured magnetic moments of NPs are in range of (1.52-2.2) mu sub(B) and almost half or less with respect to 4 mu sub(B) of bulk Fe ferrite. Magnetic anisotropy was specified as uniaxial and calculated effective anisotropy constants (K sub(eff )) are between 43.3 10 super(4) and 19.4 10 super(4) emu/g. The UV-Vis diffuse reflectance spectroscopy and Kubelka-Munk theory were used to determine the optical properties. The estimated optical band gap values (2.15-2.48 eV) of FeB sub(x)Fe sub(2-x)O sub(4) NPs are bigger with respect to reported values (1.88-2.12 eV) for Fe sub(3)O sub(4) NPs in the literature. The bigger E sub(g )values are mainly attributed to B concentration and partly to quantum confinement effect.