Microsegregation caused by the redistribution of alloying elements means the inhomogeneity of the chemical composition on the scale of a single grain. To decrease the degree of microsegregation is beneficial to avoiding ingots hot crack and shortening the homogenizing treatment time. In this study, the microsegregation of Cu element in the Al-4.5%Cu (mass fraction) alloy was investigated. Al-4.5%Cu ingots with diameter of 200 mm were prepared by the processes of conventional direct chill (DC) casting and low frequency electromagnetic casting (LFEC), respectively. The temperatures were measured during solidification processes. The effects of low frequency electromagnetic field on the microstructures and microsegregation of Cu element were investigated from eutectic analysis and electro probe microanalysis (EPMA). By the OM observations, it was found that the area fraction and dimensions of eutectic decreased markedly. The concentration profiles of Cu element were obtained with EPMA results, which showed that the profiles increased (i.e. the content of Cu element in α-Al phase increased) in the initial transit region due to the effect of low frequency electromagnetic field. The effective distribution coefficient k_e of Cu element was calculated. In the initial transit region, k_e increased to unit linearly. At the same time, the low frequency electromagnetic field made k_e a bigger value, and the value increased with the increasing of the current intensity. Because the cooling rate at the solidification front was promoted by the low frequency electromagnetic field, and there was more Cu element in α-Al, the microsegregation of Cu element in the ingot was alleviated by the LFEC process.