The precipitated grains in molten metal are to migrate due to the magnetization force. A model describing the migration of a single grain in motlen metal has been proposed in terms of the classical dynamics of rigid body, in which the grain is regarded as a sphere. The theoretical expressions about effective viscosity of the conducting melt, the velocity and distance of migrating grains were derived. The effective viscosity increases linearly with the increment of B2 (the square of the magnetic flux density). The migrating velocity is accelerated to the final value in several ms. The final speed decreases sharply with the increment of the magnetic flux density, which demonstrates that the migration of the grain is suppressed by the high magnetic field. The migration distance is closely relative with the distribution of the magnetic flux density. In order to examine the situation of the primary Si, the sample of Al-18wt%Si alloy was held at 650℃ for an hour and then a high gradient magnetic field (Bz=5T，BzdBz/dz=-224T2/m) was imposed for a certain time before quenching the sample from the melt. The experimental results indicated that the grains withradii larger than or equal to 40μm were mostly migrated in 120s，in good agreement with the theoretical calculation.