Metal oxide nanoparticles are the subject of current interest because of their unusual optical, electronic, and magnetic properties. In this work, cobalt zinc ferrite (Co0.3Zn0.7Fe2O4) nanoparticles have been synthesized successfully through redox chemical reaction in aqueous solution. The synthesized Co0.3Zn0.7Fe2O4 nanoparticles have been used for the preparation of homogenous polyvinyl acetate-based nanocomposite (Co0.3Zn0.7Fe2O4/PVAc) via in situ emulsion polymerization method. Structural, morphological and magnetic properties of the products were determined and characterized in detail by X-ray powder diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The XRD patterns of the Co0.3Zn0.7Fe2O4 confirmed that the formed nanoparticles are single crystalline. According to TEM micrographs, the synthesized Co0.3Zn0.7Fe2O4 nanoparticles had nano-needle morphology with an average particle size of 20 nm. The calculated coefficient of variation (CV) of nanoparticles diameters obtained by TEM micrographs was 16.77. The Co0.3Zn0.7Fe2O4 nanoparticles were dispersed almost uniformly in the polymer matrix as was proved by SEM technique. The magnetic parameters of the samples, such as saturation magnetization (M s) and coercivity (H c) were measured, as well. Magnetization measurements indicated that the saturation magnetization of synthesized Co0.3Zn0.7Fe2O4/PVAc nanocomposites was markedly less than that of Co0.3Zn0.7Fe2O4 magnetic nanoparticles. However, the nanocompoites exhibited super-paramagnetic behavior at room temperature under an applied magnetic field.