Abstract: Solidification of 7075 Al alloy was investigated in an alternate magnetic field with frequency being 50 Hz and density being 0.06 T, 0.09 T and 0.12 T, respectively. The Resistance-Temperature curves of the solidifications were measured with the method of double bridges to examine the liquidus and solidus temperatures of the alloy under different conditions. It is found through the experiments that, when the alloy is solidified in an alternate magnetic field, its liquidus and solidus temperatures increase obviously and the solidification interval diminishes. The stronger the magnetic density is, the more evident the effects are. Compared with the alloy solidified without a magnetic field, the liquidus temperature increases 7.5-12.5°C and the solidus temperature increases 10-20 °C by the alternate magnetic filed. It is suggested that the barrier potential required by the atom cluster jumping from liquid phase to solid phase at the front of liquid-solid interface is decreased by the alternate magnetic filed.