Fe3O4 nanoparticle was synthesized under solvothermal condition using a microwave with ethylene glycol, ethylene diamine and sodium acetate as mixture reagents. The research explored the effect of electrical power supply at different levels of 250, 400, 800 watt at temperature of 180°C on the particles sizes, morphology and magnetic characteristics in the synthetic process. The sizes and morphology of nanoparticles produced were analyzed using XRD and TEM, while magnetic characteristics were analyzed using VSM. The XRD measurement results showed that nanoparticles produced Fe3O4 crystal in the sizes between 20-50 nm in tetrahedral shapes. VSM test showed that the nanoparticles produced have superparamagnetic characteristics with magnetic saturation values of 12.7; 90.0; 97.6 emu/g, respectively. The increase of power supply used in this synthetic process will increase the transfer energy to the material, so eventually the speed of nucleatic process will be faster and the amount of nanoparticles produced will be greater. The optimum condition of nanoparticles synthesis in this research was at 180°C with 800 watt power supply, resulted in nanoparticles with relatively homogenous sizes <20 nm in tetrahedral material shapes and superparamagnetic characteristics at the saturation value of 97.6 emu/g.