To investigate the effect of interaction between polyvinyl alcohol (PVA) and iron ions (Fe2+ and Fe+3) in an aqua reaction medium on the structural and magnetic properties of magnetite nanoparticles, four different co-precipitation processes have been considered. In each process, PVA interacted with one of the cations or both of them for a designated time. X-ray diffraction patterns of all the samples confirmed the cubic spinel phase and variation of the preferred orientation peak of the structures. Scanning electron microscopy images of all the nanocomposites presented almost the same morphology. Fe3O4 nanoparticles incorporated into the PVA matrix and polydispersed cluster with ultra-small particles have been made. The magnetic measurement of the samples with the average crystallite size of 12 nm was taken by a vibrating sample magnetometer. The average size of the PVA-based nanocomposites was in the superparamagnetic region, but nonzero coercivity at room temperature was observed due to the nonuniform strain in the structure calculated by Williamson-Hall methods. Besides, Fourier transform infrared spectroscopy confirmed that four different PVA-assisted co-precipitation steps caused different ion distributions over the tetrahedral and octahedral sites of the spinel structure. Such a polymer-assisted co-precipitation method is appropriate for tuning the structural and magnetic properties of other ferrite nanoparticles that are applicable in diverse fields.