To decompose carbon dioxide, magnetite was synthesized with 0.2M-FeSO4$.$7H2O and 0.5 M-NaOH by coprecipitation. The deoxidized magnetite was prepared from the magnetite by hydrogen reduction for 1, 1.5, 2 hr. The degree of hydrogen reduction and the decomposition rate of carbon dioxide were investigated with hydrogen reduction time. The crystal structure of the magnetite was identified spinel structute by the X-ray powder diffractions. After magnetite was reduced by hydrogen, magnetite reduced by hydrogen become new phae(${alpha}$-Fe2O3, ${alpha}$-Fe) and spinel type simultaneously. After decomposing of carbon dioxide at 350$^{circ}C$, new phse(${alpha}$-Fe2O3, ${alpha}$-Fe) were removed and the spinel type only existed. The specific surface area of the synthesized magnetite was 46.69㎡/g. With the increase of the hydrogen reduction time, the grain size, the hydrogen reduction degree and the decomposition rate of carbon dioxide was increased.