In this research, magnetite (Fe3O4) has been successfully composite with graphene by a sonochemical method. The formation of the nanocomposite was confirmed by X-ray diffraction (XRD). The XRD pattern showed the magnetic phase of crystalline Fe3O4that could be attributed to the cubic structure. However, the graphene peak did not appear in the XRD pattern of the nanocomposite. The crystallite sizes of the Fe3O4 nanoparticles were estimated about 10 nm. Furthermore, the morphology of the nanocomposite was investigated using scanning electron microscope (SEM). The SEM image showed the interaction and decoration of graphene sheets with Fe3O4.Meanwhile, the synthesized graphene-Fe3O4 nanocomposite can be separated by applied an external magnetic field, thus demonstrating the magnetic properties of the nanocomposite. A working electrode was fabricated by modified a surface of carbon paste electrode (CPE) with the graphene-Fe3O4 nanocomposite. The modified electrode was characterized using cyclic voltammetry (CV) to evaluate electrochemical properties. By Modifying the surface of the working electrode with graphene-Fe3O4 nanocomposite, it can be increased the oxidation and reduction peak current that refer to excellent electron transfer materials. Therefore, the results of this research showed that the use of the graphene-Fe3O4 nanocomposite is very feasible for electrochemical sensor.