Silica coated (30 wt%) cobalt zinc ferrite (Co1 ? xZnxFe2O4, x ? 0, 0.2, 0.3, 0.4, 0.5 and 1) nanoparticles were synthesized by using sol-gel method. Silica acts as a spacer among the nanoparticles to avoid the agglomeration. X-ray diffraction (XRD) reveals the cubic spinel ferrite structure of nanoparticles with crystallite size in the range 37-45 nm. Fourier transform infrared (FTIR) spectroscopy confirmed the formation of spinel ferrite and SiO2. Scanning electron microscopy (SEM) images show that the nanoparticles are nearly spherical and non-agglomerated due to presence of non-magnetic SiO2 surface coating. All these measurements signify that the structural and magnetic properties of Co1 ? xZnxFe2O4 ferrite nanoparticles strongly depend on Zn concentration and nanoparticle average crystallite size in different Zn concentration regimes.