The magnetic field microgravity effect is the phenomenon that a secondary microgravity is produced when a magnetic field is introduced into an electricconducting liquid, leading to the decline or extinction of the natural convection driven by buoyancy. The effect level could be described by the formula gm＝（β0/βm）（ν0 /νm）2　g0, according to the change of Grashof number. If neglecting β changes, the formula is simplified to gm＝（ν0/νm）2 g0, by which the effect levels corresponding to different horizontal magnetic field strengths were estimated on the basis of the measurement of the viscosities of silicon melt under different conditions of the magnetic field. The results showed that the estimated results agree with the experimental data.