Abstract: The effect of frequency of LF electromagnetic field on microstructure of Φ100 mm semi-continuously cast Al alloy ingot was investigated by metalloscope and TEM. The results show the grain size decreases somewhat, but number of equiaxial grains increases with a current 100 A at 15 Hz as compared with conventional ingot, grain refinement is found to be most obvious at 25 Hz. But grain size turns to increase then with increasing frequency. With a current 100 A at 25 Hz, the intergranular dislocation density becomes higher with smaller size of precipitates which are mainly granular η', η and lamellar T phases and the precipitate-free zones on both sides of grain boundary are about 100 nm wide. However, the size of intergranular precipitates increases somewhat at 15 Hz, which are mainly lamellar or lath η, lamellar T and granular η' phases with precipitate-free zones about 300 nm wide.