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The nucleation and growth behaviors of primary Al phase in Al-20.8wt%Cu hypoeutectic alloy in high static magnetic fields were investigated with aid of differential thermal analysis (DTA).The DTA curves indicated that the nucleation temperature of primary Al phase decreased as the magnetic field intensity increased.The average growth rates of primary crystals increased with magnetic fields.The dendrite structures showed that primary Al phase dendrites changed from disorderly without the magnetic field to regularly with the field.The reduction of nucleation temperature of primary Al phase was mainly caused by increase of interfacial free energy between the melt and nucleus.The change in dendrite morphology could be attributed to suppression of melt flows in the magnetic field and magnetic anisotropy of Al crystals.