Magnetic fluid seal is widely used in military and aerospace industry owing to its zero leakage property, long life and reliability. The resistance torque is often required to be small and stay within allowable scope with the
standing time expanding and the working environment changing. In this paper, we discussed the influence of the particle size distribution of magnetic fluid on the resistance torque of magnetic fluid seal. Samples of magnetic fluid of different particle size distribution were processed by applying a magnetic field gradient. Afterwards, the rheological property of different samples was analyzed using rheometer. Besides, the resistance
torque of magnetic fluid seal on different occasions was measured. We explained these results from the point of formation and destruction of magnetic aggregations based on the theory of magnetoviscous effect, thus  provided feasible ways to improve the performance of magnetic fluid seal.