We have synthesized iron oxide nanoparticles in air atmosphere by coprecipitation. The ratio of was fixed at 2/3. The total amount of iron ions in the solution was varied from 250 mmol to 12.5 mmol. X-ray diffraction (XRD) patterns showed the characteristic peaks of iron oxide phases. We observed that the crystallinity reduced and particle sizes calculated from XRD patterns decreased as the iron ion concentration in the solution decreased. Fourier transform infrared spectroscopy analysis exhibited the Fe-O vibration band at 560-580 confirming the iron oxide formation. The mean physical sizes obtained from a transmission electron microscope are around 8 nm. According to the magnetic measurements, all samples are superparamagnetic at room temperature showing an increase in saturation magnetization up to 63.71 emu/g. Magnetic sizes of the particles vary from 7.45 nm to 4.88 nm with the change of iron ion concentration.