Spinel zinc ferrite (Zn1-xCoxFe2O 4) nanoparticles with various particle sizes were prepared by the microwave combustion method using urea as a fuel. The composites were prepared with the addition of cobalt at different molar ratios (x=0.0 to 0.5) to ZnFe2O4. The obtained spinel ferrites were characterized by X-ray powder diffraction (XRD) and their mean grain size and morphology were determined by the high resolution scanning electron microscopy (HR-SEM). The magnetic properties of the synthesized ferrites were investigated using room temperature vibrating sample magnetometer (VSM) and their hysteresis loops were obtained. The optical reflectance and photoluminescence (PL) emissions were determined by UV-visible diffuse reflectance spectra (DRS) and PL spectra respectively. The formation of single cubic spinel phase was confirmed by XRD and Rietveld analysis with an average crystallite size is in the range of 43-49 nm. The broadband visible emission band is observed in the entire PL spectrum and the estimated energy band gap is about 2.1 eV. The variation of saturation magnetization (Ms) value of the samples was studied. The prepared lower compositions (0.0, 0.1 and 0.2) show a superparamagnetic behavior and the higher compositions (0.3, 0.4 and 0.5) show a ferromagnetic behavior with hysteresis and that the Ms increases with increasing Co content to reach a maximum value of 65.20 emu/g for Zn0.5Co0.5Fe 2O4. The relatively high Ms of the samples suggests that this method is suitable for preparing high-quality nanocrystalline magnetic ferrites for practical applications. Different mechanisms to explain the obtained results and the correlation between magnetism and structure are discussed. © 2013 Elsevier B.V. All rights reserved.