The solidification of Al–20 wt% Mn alloy was investigated under pressures up to 6 GPa. It was found that the solidification products under pressures below 4 Gpa were about the same, composed mainly of Al and Al6Mn. A new Al–Mn phase with needle like morphology and Al nanocrystallites in size less than 20 nm were obtained in the quenched alloy at 6 GPa. Structure analysis by transmission electron microscopy and x-ray diffraction indicated that the new phase had a C-center othorhombic unit cell with lattice constants of a = 0.7565(4) nm, b = 1.2965(6) nm, and c = 0.7801(6) nm. The composition was determined to be Al77.5Mn22.5 by election probe microanalysis. The phase evolution during solidification under different pressures was discussed. Our experimental results show that the pressure, as a basic thermodynamic variable like temperature, may play an important role on the solidification of the alloy.