Thermal decomposition process was applied to synthesize Fe3O4 and Nd2Fe14B nanoparticles which were prepared by reduction diffusion process for preparing exchange- coupled nanoparticles. The magnetic Nd2Fe14B nanoparticles used in this study as the starting materials were synthesized via metal oleate complex method. Also mechanical ball mill technique was applied to the mixture of magnetic Nd2Fe14B and alpha-Fe nanoparticles to build an exchange- coupled nanoparticle. The mixture of Nd2Fe14B= alpha-Fe of nanoparticle was confirmed by transmission electron microscopy ( TEM). The crystal structure of nanoparticle was characterized by X- ray diffraction pattern ( XRD). The magnetic properties were characterized with saturation magnetization from hysteresis loop by vibrating sample magnetometer ( VSM). Thermogravimetry using a microbalance with magnetic field gradient positioned below the sample was used for the measurement of a thermomagnetic analysis ( TMA) curve showing the downward magnetic force versus temperature.