Abstract: Directional solidification and quenching experiments are conducted on hypoeutectic Mn-89.7%Sb(w) alloys under different magnetic field conditions. The influence and mechanism of gradient high magnetic fields on the growth behaviors of primary phases and microstructure evolutions of directionally solidified alloys are investigated. When the magnetic field is 0 T, the primary MnSb phase grows directionally with a well-developed dendritic morphology. When the high magnetic field is applied, the primary MnSb phase transitions from a developed dendritic morphology to an irregular fine dendritic structure, and an irregular block-shaped MnSb/Sb eutectic structure appears in the alloy. The primary MnSb phase exhibites crystal orientation with its c-axis perpendicular to the direction of the magnetic field. The mechanism of the high magnetic field on the quenching micro-structure of the alloys is further explored. It is found out that during the directional solidification, the thermoelectric magnetic force-induced convection at the solid/liquid interface causes lateral migration of solute and fragmentation of the primary MnSb phase. The magnetic force drives the migration of the Mn-rich and Sb-rich zones, as well as the primary MnSb phase fragments, along the longitudinal direction at the solid/liquid interface. The competition between these two forces leads to the evolution of the microstructure of the primary MnSb phase.