The effect of reaction temperature (T-R, ranging from 40 degrees C to 80 degrees C) on magnetic hyperthermia measurements was investigated in the surfactant-free co-precipitation synthesis of magnetite (Fe3O4) nanoparticles (NPs). The size, structure and magnetic properties of the synthesized NPs were studied by X-ray diffraction, field-emission scanning electron microscopy (together with energy dispersive spectroscopy), hysteresis loop and first-order reversal curve analyses. The investigations showed that all the NP samples were single domain with a super-paramagnetic (SP) feature. After preparing the respective ferrofluids with a concentration of 5 mg/ml (in distilled water medium) acting as nanoheaters, hyperthermia measurements were performed under an alternating magnetic field with a frequency of 400 kHz. It was found that T-R = 60 degrees C resulted in the maximum specific loss power (SLP) value of 181 W/g together with an SP fraction of 82%, being a suitable candidate for use in magnetic hyperthermia. In this case, the resulting saturation magnetization and SP fraction were in the middle of corresponding values obtained from T-R = 40 degrees C and 80 degrees C. Our results showed that a balance between relatively high saturation magnetization and low coercive field led to the maximum SLP.