In this study, magnetite (Fe3O4) nanoparticles were synthesized at room temperature using FeCl3.6H2O (1.28 M), FeCl2.4H2O (0.64 M) and HCl (0.4 M) for preparing a solution as the iron source and NaOH (0.9-1.5 M) for to prepare a solution as the alkali source by the aqueous phase co-precipitation method under vigorous mechanical stirring (450 and 750 rpm) together with N2 gas flowing through the reaction medium during synthesis operation in closed system. The powder samples were characterized by the commonly used techniques of scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infra-red (FTIR), X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and BET analyses. SEM was used to observe the morphology and agglomeration state of the powder. Size and morphology of the precipitated crystallites were examined with TEM. The prevalence of functional groups in the synthesized powders was ascertained by FTIR spectroscopy. The pure magnetite and other phases according to processing parameters were observed by XRD analysis. The magnetic properties of magnetite (Fe3O4) nanoparticles were examined using VSM. Finally, the specific surface area of nanoparticles was measured by BET technique. The results indicate that smaller particles can be synthesized by increasing stirring rate and decreasing the NaOH concentration, which in this case corresponded to 35 nm using 0.9 M NaOH at 750 rpm. The VSM analysis showed a saturation magnetization range of (82-96 emu/g) and coercivity of (83-119 Oe) for particles between (35-96 nm) respectively. Also, the highest specific surface area of 40 m2/g was obtained at 0.9 M NaOH at 750 rpm and the smallest value of 15 m2/g at 1.5 M of NaOH at 450 rpm using BET analysis.