Abstract: To transmission microscope, laser scatter scanner, M ossbauer spectrometer and atomic force microscope are used to determine particle size of magnetic iron oxide solution. The results show that particle size distribution of the magnetic iron oxide solution prepared is between 5-100 nm. The most of the particles are with the size less than 20 nm by laser scatter scanner and atomic force microscope. So the magnetic iron oxide solution belongs to nano-particles,but the measuring sizes are some difference by the four methods.