Abstract: The solidification structure of Cu-40%Pb monotectic alloy was experimentally investigated in a 12 T high magnetic field, including the coexistent monotectic morphology of solidification. The results showed that during the solidification in such a magnetic field the rod-like and regularly arranged extra-long Pb-rich phase forms at the centre of specimen. The high magnetic field enables the Pb-rich phase in solidification structure to have a elongating effect, so the percentage elongation is efficient to analyze the result from the elongation effect. The high magnetic field can also hold back the motion of Pb-rich phase droplets and the melt flow along the solidification front, which is just the reason why the extra-long and regularly arranged monotectic structure forms.