Aiming to develop an effective solidification purification process by Al-Si alloy refining method, experiments under rotating electromagnetic field (REMF) with varying frequencies have been carried out. High B/P removal rates under REMF can be confirmed. A primary Si-rich layer formed near the side surface of the sample, while the interior of the sample is mainly eutectic Si/Al. Effect of varying frequency of REMF on alloy structure, size of the primary Si flakes and B/P contents in the primary Si flakes are studied. An apparent segregation coefficient is introduced to describe refining process. The apparent segregation coefficients of B and P are determined to be 0.12 and 0.06 respectively when the frequency of REMF and the cooling rate are10 Hz and 50 mK s(-1) respectively.