Nanotechnology has been shown as an important tool for developing intelligent devices. In particular, magnetic nanoparticles have been studied due their applications in cancer treatments. However, nanoparticles need to be tightly controlled in relation to size, shape and coating. It makes particles suitable for in vivo applications. In this work, magnetite nanoparticles were used for particle size characterization. Fe3O4 crystals were coated by polyethyleneglycol (PEG 4000). Three different techniques were performed to obtain the nanoparticles average diameter: Rietveld Analysis, Scherrer Equation and Nanosight®. Results indicate good properties.