The carbon graphite encapsulation of metal alloy magnetic nanoparticles has been attracted attention for biological application due to their high magnatization. However, most of the synthetic methods have limitations in terms of scalability and economics owing to the demand synthetic conditions and low yields. Here, we show that well controlled carbon graphite encapsulated FeCo core-shell nanoparticles can be synthesized by hydrothermal method simply mixing iron-cobalt and sucrose as a source of carbon. Various metal ratios between iron and cobalt were produced to determine the compositional dependence of the saturation magnetization and relaxivity coefficient. In addition, we demonstrated that the application for the magnetic resonance imaging (MRI) contrast both in vitro and in vivo and the combination of FeCo/C magnetic nanoparticles-based siRNA delivery against the oncogenic receptor (EGFRvIII) with hyperthermia using the same magnetic nanoparticles results in a synergistic inhibition of brain-tumor cell proliferation and induction of apoptosis.