A magnetic annealing effect has been observed in a ferromagnetic MnBi single crystal. It was found that by heating an MnBi single crystal in a magnetic field perpendicular to the easy axis of magnetization of the crystal, the easy axis of the magnetization and the hexagonalcaxis of the crystal would change into the direction parallel to the field through a recrystallization process induced by the magnetic annealing. This recrystallization phenomenon is similar to that observed by Boothbyet al.on a polycrystalline sample. Based on the microstructure investigation and transformation kinetic analysis, it was found that the mechanism of the recrystallization in a single crystal is a thermally activated nucleation and interface controlled growth process. It was established that the magnetocrystalline anisotropy energy of the crystal is responsible for the recrystallization.