This study aims to synthesize and characterize Fe3O4@SiO2 sub-nanoparticles (SNPs) with high saturation magnetization (SM). The research process was conducted in simple and environmentally friendly conditions. The results of ultraviolet-visible (UV-Vis) spectroscopy and X-ray diffraction (XRD) analysis presented that the Fe3O4@SiO2 SNPs were well formed and the phase change of Fe3O4 NPs did not happen in Fe3O4@SiO2 SNPs. Transmission electron microscope (TEM) analysis showed that the Fe3O4@SiO2 SNPs are in a fairly spherical shape with a core/shell structure and a diameter in a range of 100 nm to 500 nm. Fourier transform infrared spectrometry (FT-IR) spectra of Fe3O4@SiO2 SNPs presented some absorption peaks indicating the existence of Si-O-Si, O-Si-O, Fe-O and Fe-O-Si. The SM of Fe3O4 particles and Fe3O4@SiO2 SNPs determined via vibrating sample magnetometer (VSM) were 50.9 emu.g−1 and 19.5 emu.g−1, respectively. All the above results provide clear evidence that the Fe3O4 particles were coated by SiO2 to form sub-nano core/shell with great SM.