Zn substituted cobalt ferrite nanoparticles having formula of ZnxCo1-xFe2O4 (x = 0.0-1.0) were prepared by hydrothermal technique. In this process, triethylene glycol was used as a solvent and surfactant, anhydrous sodium hydroxide was used as an alkalinity additive. Investigation of the structural, morphological and thermal properties were carried out using X-ray diffraction (XRD), infrared spectroscopy (FT-IR), transmission electron spectroscopy (TEM) and thermal analysis (TGA) respectively. The X-ray diffraction study reveals that the lattice constant of cobalt ferrite increases with the increase of Zn content. Magnetization measurements showed that as zinc concentration increases saturation magnetization initially stays constant and then decreases monotically. Samples having high zinc concentration (x >= 0.6) have superparamagnetic behavior like absence of saturation at high fields, low saturation magnetization values and immeasurable coercivity. These features were explained by surface spin disorder and canted spins. (C) 2012 Elsevier Ltd. All rights reserved.