The influences of low frequency electromagnetic field on microstructures and macro-segregation in direct chill casting process were investigated in the experiments，Al-19.2%Si alloys were semi-continuously cast into ingots with 100 mm in diameter. Microstructures and macrostructures of samples taken from different part of the cast with different electromagnetic field conditions were characterized by optical microscopy. The results showed that low frequency electromagnetic field refines the microstructures and reduces macro-segregation. Unlike casting without electromagnetic field, the primary Si grains were homogeneous with fine dimensions and the morphology of the primary silicon exhibited small blocky structures or near-spherical structures. Further more, decreasing frequency is beneficial to the improvement. In the range conditions in the experiments, the optimum frequency is found to be 15Hz. During casting, the temperature of the liquid metal in the sump was monitored. The results shown, under LFEC the width of the liquid-solid region became narrowed and the temperature field in the sump also became homogeneous.