Magnetic hyperthermia is a non-invasive or minimally invasive method for cancer treatment in which super-paramagnetic nanoparticles are used. In this project, effect of hydrothermal process parameters on physical, magnetic and thermal properties of Zn-substituted magnetite nanoparticles (Zn 0.3 Fe 2.7 O 4 ) were assessed. The magnetic nanoparticles were synthesized via hydrothermal-reduction route in the presence of citric acid, at temperatures of 150, 175 and 200 °C for duration of 10, 15 and 20 hours. The purity, chemical composition, microstructure, magnetic and thermal properties of samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and specific loss power (SLP) measurements, respectively. The results showed that temperature and time of the hydrothermal process had a significant effect on chemical composition and different properties of the synthesized nanoparticles. It was observed that at 150 °C, heat generation was not enough to produce the activation energy required for nucleation of Zn 0.3 Fe 2.7 O 4 spinel nanoparticles, even after a long time. At 175 ° C, although temperature is low, but the suitable condition for nucleation of nanoparticles was made and spinel nanoparticles with the size of about 13 nm were formed after 15 hours. Nonetheless, since crystallinity and specific loss power of the nanoparticles was low, they showed weak performance for magnetic hyperthermia. At 200 ° C, although activation energy was provided for nanoparticles nucleation, due to the high temperature, the spinel was oxidized to hematite, resulting in decrease in thermal and magnetic properties of the nanoparticles. In overall, the nanoparticles made have at 200 ° C for 15 hours was chosen as the best sample having a reasonable purity, magnetic and thermal properties (SLP= 18.71 W/g). In order to increase biocompatibility of the synthesized nanoparticles, their surface was coated by polyethylene glycol (PEG) polymer. Fourier transform infrared spectroscopy (FTIR), SEM and SLP measurements were conducted to investigate the formation of polymer coating on the nanoparticles surface, morphology, size distribution and thermal properties of the coated nanoparticles. The results showed that the PEG were successfully formed on the nanoparticles surface and of the nanoparticles and their shape was still spherical. The SLP of the coated nanoparticles (17.62 W/g) showed no significant difference to that of the uncoated nanoparticles. Keywords : Zn 0.3 Fe 2.7 O 4 magnetic nanoparticles, magnetic hyperthermia, hydrothermal-reduction route via citric acid, polyethylene glycol (PEG) polymer, coating.