The application of magnetic fluid in ferrography has improved the technique for separating nonferrous wear debris by magnetic means. Brass debris was used as an example to analyze the magnetization mechanism based on ferrographic experiments and molecular analysis. The results show that the nanomagnetic particles in magnetic fluid attach to the surface of the micro nonferrous debris by the functional group of surfactant molecule so as to develop in that debris a positive susceptibility and make it responsive to the magnetic field. The magnetic action of the magnetic particles is selective as to the property of the debris surface to which it attaches, and the magnetization time depends on how readily of the attachment. Mathematical derivation based on mechanical models shown that the magnetic force exerted on the debris is strong enough for its precipitating on ferrogram.