Thermoelectric magnetic force (TEMF) imposed on the liquid/solid interface and the cell was examined numerically and experimentally during the directional solidification of the Al-Cu alloy under an axial strong magnetic field. Numerical results reveal that the TEMF is related to the magnetic field intensity, temperature gradient and liquid fraction. Indeed, it has been found that the TEMF increases as the magnetic field intensity, temperature gradient and liquid fraction increase. In particular, the value of the TEMF in directional solidification at 60 K/cm temperature gradient under the 10 T strong magnetic field is in the order of 10(5) N/m(3). This force is strong enough to induce the Asaro-Tiller-Grinfield (ATG) instability. Experimental results indicate that the application of the strong magnetic field in directional solidification has led to a dramatic change in the Mullins-Sekerka (MS) instability and the deformation of the solid. The modification amplitude of the MS instability under the magnetic field is in good agreement with the value of the TEMF imposed on the solid. Therefore, the change in the MS instability should be attributed to the occurrence of the ATG instability induced by the TEMF. Moreover, the experimental result may act as an evidence for the strong coupling between the MS and ATG instabilities in directional solidification. (C) 2011 Elsevier BM. All rights reserved.