Abstract: The plot of viscosity versus particle volume fraction for the water carrier of self-formed CoFe2O4 magnetic fluid is abnormal in zero magnetic field. However，the viscosity theory of the suspension with the global rigid particle filling cannot explain the experiment well. That is because the nanoparticles have aggregated before preparation of magnetic fluid. The fact is found that the sedimentation without magnetic field and the becoming chains in magnetic field of this type of magnetic fluid need the big particles which core are pre-aggregates by researching the interaction of particles of magnetic fluid. Around the big particles，nanoparticles are absorbed with the type of dynamic state. It is on that idea that the model of fluctuant aggregation is made. So，the average diameter，Einstein ratio and particles size distributive deviation of free suspended bodies in zero magnetic fluid are the functions of the particles volume fraction. And then，Popplewell’s formula of the viscosity is modified with this model. As a result，a well-fitted curve is obtained.