Relatively pure Fe3O4 nanoparticles was synthesized by microwave hydrothermal (MWH) and sonication-assisted co-precipitation. The precipitating agent was 0.5 % hexamine in 10 % ammonia solution. This was added to 100 mL of 0.01 M HCl containing stoichiometric amount of stable salt of Fe3+ and Fe2+ ions. The formaldehyde produced from hydrolysis of hexamine prevents partial oxidation of Fe2+ during the precipitation process. For stabilization of colloidal suspension a small amount of a bifunctional fatty acid was used to coat the Fe3O4 nanoparticles and make them hydrophobic. The extraction of Fe3O4 nanoparticles from aqueous phase into chloroform gave a stable magnetite suspension which was attracted by a magnet. The product was analyzed by XRD, LLS, EDX and quantities chemical analysis of iron content in magnetite. The average diameter of nanoparticles was found to be about 6.2 nm. The order of main factors according to their effectiveness on the yield of product was determined using a factorial design and ANOVA