Indonesia as a maritime country has many lands and beaches which own many iron sands. This abundant amount of natural iron sand can be used as the main source of ingredients to produce magnetic materials that have many advantages. One of the advantages is as raw material for synthesizing Mn_0.25Fe_2.75O₄/Activated Carbon (AC) magnetic nanoparticles which can be used for the material in absorbents or in the formation of a capacitor. The Mn_0.25Fe_2.75O₄/AC magnetic nanoparticles have been successfully synthesized by using the co-precipitation method. Furthermore, the variations of AC mass in Mn_0.25Fe_2.75O₄/AC nanoparticles also have been investigated to find out its effect on the nanostructural and magnetic properties of the Mn_0.25Fe_2.75O₄/AC nanoparticles that are characterized using X-Ray Diffraction (XRD) and Vibrating Sample Magnetometer (VSM) instrument respectively. The particle sizes of the Mn_0.25Fe_2.75O₄/AC with 0.1, 0.2, and 0.3 g variations of AC filler characterized by using XRD were 7.4, 3.67, and 11.8 nm, respectively. Based on VSM characterization, the values of saturation magnetization of the Mn_0.25Fe_2.75O₄/AC were 0.77, 0.003, and 0,002 emu/gram with the AC mass variations of 0.1, 0.2, and 0.3 g, respectively.