Magnetic iron oxide (Fe3O4) and Fe3O4 @SiO2 core-shell were prepared via co-precipitation and modified Stöber methods. Structural, morphological, and optical properties were examined using XRD, FTIR, and UV–visible spectrophotometer. This study used three different methods to deduce the studied nanoparticles' optical gap energy: (i) the Tauc model, (ii) the absorption spectrum fitting model, and (iii) the derivation of the absorption spectrum fitting model. The calculated values of gap energy of Fe3O4 @SiO2 NPs by the three different methods are higher than the corresponding values of Fe3O4 NPs and SiO2 NPs. The optical oscillator strength (f) of Fe3O4 @ SiO2 NPs has the greatest value (24.4 eV2) compared to the others. The values of the high-frequency dielectric constant (ε∞) are greater than the square of the infinite wavelength refractive index (n2). Therefore, the free charge-carrier contribution during the incidence of photon energy on the tested NPs significantly affects the polarization. The high value of the third-order nonlinear optical susceptibility (χ(3)) of Fe3O4 @ SiO2 NPs can candidate them to be applied in the applications of nonlinear optics as power limiters. The nonlinear absorption coefficient (βc) increased by increasing the incident energy and has a maximum value in a range of energies from 3.25 eV to 5 eV, for all the samples. The synthesized Fe3O4 @SiO2 core-shell nanomaterials with enhanced optical parameters is considered as a suitable candidate in nonlinear optical applications.