Co-precipitation from a solution of ferrous/ferric mixed salt with the ratio of Fe2+:Fe3+ = 1:2 in air atmosphere is not a reliable method to synthesize magnetite (Fe3O4) nanoparticles because of the fact that Fe2+ oxidizes to Fe3+ and the molar ratio of Fe Fe2+:Fe3+ changes. Therefore, the phase composition changes from magnetite to maghemite (gamma-Fe2O3). The influence of the initial molar ratio of Fe2+:Fe3+ on the phase composition of nanoparticles, their crystallinity and magnetic properties was Studied. Experimental data from XRD, FTIR, SEM, and VSM reveal that the appropriate method to synthesize magnetite nanoparticles is reverse precipitation from only ferrous salt. It is found that by decreasing the synthesis temperature and by increasing the concentration of alkaline solution and the ratio of Fe2+:Fe3+ the crystallinity and the specific saturation magnetization (sigma(s)) are increased.