A one-step sonochemical synthesis of a uniformly dispersed aqueous solution of amorphous Fe3O4 nanoparticles capped with DL-cysteine (cys) has been achieved. During the sonication, several processes or chemical reactions occurred: the decomposition of iron pentacarbonyl (Fe(CO)5), the formation of amorphous Fe3CO4 nanoparticles, the capping of the Fec with cysteine molecules, the formation of a surface chemical bond of Fe-S between Fe3O4 particles and sulfur from cysteine, and the formation of cystine by linking two cysteine molecules. These capping cystine and cysteine molecules acted both as antioxidants and stabilizers, and the capped Fe3O4 colloids could be stable as long as 3 months at room temperature. Usually, the size of the amorphous Fe 3O4 nanoparticles capped with Cys was found to be 20-30 nm. The products were characterized by a number of analytical techniques, such as XRD, DSC, TGA, XPS, TEM, HRTEM, Raman Spectroscopy, magnetic measurements, and spot tests.