• The first time applying in situ synchrotron imaging to Al-Cu alloy with La-Ce. • The real time solidification of Al-Cu alloy with La-Ce has been captured. • La-Ce decreases nucleation density of α-Al at a lower cooling rate. • La-Ce leads to grain refinement of α-Al at a higher cooling rate. The effect of La-Ce addition on the solidification behavior of hypoeutectic Al-Cu alloy was investigated using in situ synchrotron X-ray imaging with conventional casting experiments. The results show that La-Ce addition leads to a decrease in nucleation density and coarser grains at a lower cooling rate since La-Ce addition can suppress α-Al nucleation to a certain extent. At a higher cooling rate, melt undercooling can be increased by inhibiting early nucleation of α-Al on heterogeneous particles with large size, resulting in grain refinement.