Quantum size effects and the large surface area of magnetic nanoparticles dramatically change some of the properties and exhibit “superparemagnetic”phenomena. The magnetization behavior of these particles, above a certain temperature (Blocking temperature) is identical to that of atomic paramagnets (super para magnetism) except that an extremely large moment and thus large susceptibilities are involved. All biomedical and bioengineering applications require that these nanoparticles have high magnetization values and size smaller than 100 nm with overall narrow particle size distribution, so that the particles have uniform physical and chemical properties. Since particles are attracted magnetically, in addition to the usual flocculation due to Van der Waals force, surface modification is often coating is desirable. In these study magnetite nano particles magnetite particles (Fe 3 O 4 ) with a size under 100 nm were prepared by the modified controlled chemical co-precipitation method from the solution of ferrous and ferric mixed salt –solution in alkalin medium. In the process of synthesis, several coats added to nanoparticles and their effects on the magnetic properties of the nano particles were studied.We studied surface derivatization of magnetite by fatty acids (oleic acid, linoleic acid and palmitoleic acid) and one polymer (starch).These coats are biocompatible and are chemisorbed on the surface of the nanoparticles, which makes the particles hydrophobic; thus they become dispersible in nanopolar solvents for biomedical applications. The magnetit nanoparticles were characterized by x-ray diffraction analysis (XRD).the XRD spectra of coated iron oxide nanoparticles exhibited peaks that correspond magnetite(Fe 3 O 4 ). We determined the size and size distribution for samples by SEM images and by particle size analyzer (ZETA- SIZER).The mean magnetice diameter was less than 100 nm .our samples show in the absence any surface coating . magnetic iron oxide particles have hydrophobicsurfaces with a large surface area to valume ratio. Due to hydrophobic interactions between them and show ferromagnetic behaviour. TGA (Thermo gravimetric Analysis) was performed to analyze the surface characteristic of the nanoparticles. The TGA indicated chemisorptions of coats at the iron oxide nanoparticle surface. TGA derivative curves show two distinct transitions for all of the samples. An estimate of the average number of coat molecules on a particle was obtained from the area of the second weight loss peak relative to the final residual sample weight. The magnetic properties of the resultant Fe 3 O 4 nanoparticles were measured with a vibrating sample magnetometer (VSM, Quantum Design) at room temperature. Datas show that our samples are superparmagnetic with no coercivity and remanence.