Magnetite (Fe3O4) nanoparticles (NPs) with various morphologies obtained by varying solvent (water)-to-surfactant (polyethylene glycol, PEG) ratios have been investigated for magnetic hyperthermia (MHT) application. The water-to-PEG ratio influenced the size, shape, and chemical composition of the NPs, which changes their magnetic properties and hyperthermia (HPT) response. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies reveal the formation of well crystalline inverse spinel structure of Fe3O4 with some Fe0 phases. Morphology of the NPs varies from nearly spherical to elongated to pseudohexagonal to cluster with an increased concentration of PEG. The NPs possess enhanced saturation magnetization ranging from 90 to 98 emu/g. The HPT studies indicated that the NPs show an enough specific absorption rate (SAR) under alternating magnetic field suitable for biological application with hexagonal Fe3O4 being more efficient. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.