Abstract A magnetic field generated by a magic ring was applied to the space containing the silicon melt, and the effective viscosities of silicon melt were measured by the method of rotary vibration. The magic ring was constructed by permanent magnets made of NdFeB permanent-magnet material. The results showed that, at certain temperature, the effective viscosity increased with the increase of magnetic field intensity, and they are parabolically related. The magnet field had stronger effect when the silicon melt temperature increased, which was indicated by a more warped up parabola. In the range of 1490 to 1610℃, the viscosity had an extraordinary fluctuation which indicated a structure change in the silicon-melt. The value of silicon melt viscosity increased by 2—3 orders of magnitude when the magnetic field reached 0.068T. The results indicated that applying a magnetic field to the silicon melt space is an effective way to suppress thermal convection during the growth of large diameter single crystal silicon.