Manganese (Mn)-doped black iron oxide (Fe₃O₄) magnetic fluids in the system of Mn_xFeO₄ were successfully synthesized from natural magnetite (iron sand) by using co-precipitation method at room temperature. The analyses of the small angle neutron scattering (SANS) data by applying a log-normal sphere with a mass fractal models for and and two log-normal spheres with a single mass fractal models for , 0.75 and 1 revealed that the primary particles of the Mn_xFeO₄ fluids tended to decrease from 3.8nm to 1.5nm along with the increasing fraction of Mn contents. The fractal dimension () increased from about 1.2 to 2.7 as the Mn contents were increasing; which physically represents an aggregation of the Mn_xFeO₄ particles in the fluids growing up from 1 to 3 dimensions to consolidate a more compact structure. The magnetization curves of the fluids exhibited an increasing saturation magnetization from to , and a decreasing on and 0.75, with the maximum achievement of . These phenomena may probably be due to the combined effects, arising from cationic and dopant distributions, aggregation and its size, and also fractal dimension. Furthermore, the decrease of blocking temperature of the Mn_xFeO₄ magnetic fluids could be associated with the reduced particle sizes, while the freezing temperature had its highest peak intensity when it collectively occurred with the blocking temperature at a similar point of about 270K.