Iron oxide (Fe3O4) nanoparticles with average sizes of 10 nm were synthesized by a chemical coprecipitation method in the presence of chitosan. Chitosan as a natural polymer which can be extracted from crustaceans was used in the synthesis process in order to achieve more dispersed nanoparticles. Also, chitosan was used to obtain functionalized magnetic nanoparticles for using in different area of research. Chitosan should be first carboxymethylated and then covalently bounded onto the surface of Fe3O4 nanoparticles via carbodiimide activation. Because of size – dependent different property of nanoparticles, The effects of the carboxymethyl chitosan concentration, sonication time and the reaction temperature in the synthesis procedure of chitosan-bound Fe3O4 nanoparticles were investigated and then the best condition used for final product. Dynamic light scattering were used to investigate the effect of each experimental parameters on the size and size distribution of prepared nanoparticles. Transmission electron microscopy (TEM), dynamic light scattering (DLS), UV-Vis and FT-IR spectroscopy, vibrational sample magnetometer (VSM) and zeta-potential methods were carried out on the final productin order to characterize the size, size distribution, magnetic property and surface charge of as-prepared nanoparticles.