Abstract: Al-10%wt Si alloy made of high purity aluminum and silicon(>99.99%wt) is modified by Na-Salt.The result from analuyzing.the eutectic Si-phase indicates that the Si-phase lags behind the Al-phase when Al-Si alloy eutectic solidified.modification not only changes the laminar Si-phase to fibrous Si-phase whose solidification lags more than the formar's,but also increa ses greatly the ednsity of twin crystal in Si-phase.The twin crystalaxis is(111)_si.The crystal orientation is(211)_si.The two phas,Si-phase and Al-phase in eutectic,accord with a regulation that100 al parallels to110_si.Further more,the modified eutectic Si-phase branches,refines,flexes in a ccordance with TPRE growing-mechanism,which is an importat approach to form modified crystallization structure.