The rupture forces of breaking down a magnetite chain in a uniform magnetic field were measured by using a water flow under microscope observation with a video camera. The experimental results have shown that there was a linear relationship between the rupture force and the field intensity, which was expressed by F-M = kH. The value of the slope (k) was dependent on the magnetite particle size in the chain. It was found that the rupture force was very weak in a low-intensity magnetic field, around 0.01-0.1 mN, indicating that the chains must be kept in a very weak turbulence of suspension.