The optimum conditions was synthesized for the formation of Magnetite ($Fe_3O_4$) by air bubbling with the suspensions obtained by mixing Ferrous sulfate ($FeSO_4cdot 7H_2O$) and Sodium Hydroxide (NaOH) solution in various values equivalent ratio($R=2NaOH/FeSO_4$) were studied. The changes of the structure were measured with XRD, $EM and BET. Equivalent ratio R: 0.65 was synthesized Goethite ($alpha$-FeOOH), which becomes Maghemite ($gamma=Fe_2O_3$) by dehydration, reduction and oxidation process. At the equivalent ratio over 1 (R>1), Magnetite ($Fe_3O_4$) was synthesized directly. The oxygen-deficient Magnetite ($</TEX>Fe_3O_{4-delta}$), which is obtained by flowing $H_2$ gas(100 ml/min) through the synthesis Magnetite at 350$circ$C for 4 hr. By using it, was researched the decomposition reaction of $CO_2$. $CO_2$ was decomposed nearly 100% in 45 minutes by the oxygen-deficient Magnetite.