We made magnetite which was prepared with mixed liquids of Fe$^$2+/and Fe$^$3+/ added to the 3N-NaOH by coprecipitation reaction. Ahead of making magnetite, we investigated variation of physical properties for changing Fe$^$2+/ : Fe$^$3+/. Through the variation of the process temperature, we examined physical properties of magnetite. Also, to examine possibility of magnetic fluids, from magnetite manufactured in this way, we examined the crystal structure by X-ray diffraction pattern. The saturated magnetization $sigma$$\_$s/, value has 60.8 emu/g, when Fe$^$2+/ to Fe$^$3+/ ratio is 0.46 : 0.54 in the synthetic magnetite. Keeping the condition, when the synthetic temperature is kept at 80$^{C}$ for 30 minutes, we get $sigma$$\_$s/, value 63.51 emu/g. It shows that $sigma$$\_$s/, value is bigger as the synthetic temperature is higher.