With the multiphase field method,the stability of lamellar basic state is investigated during the directional solidification of eutectic alloy CBr4-C2Cl6.A great number of lamellar patts observed in experiments are simulated,and a stability diagram for lamellar patt selections is presented.The simulated growth behaviours of these patts are found to be qualitatively consistent with Karma et al’S numerical calculations and experimental results.The formation of the primary instability is attributed to the destabilization of solute boundary layer.