A new concept of fragility of superheated melt is used to analyze the viscosity of eutectic Al-12 wt%Si melts modified with phosphorus (P). The influence of P on the melt is investigated on the basis of Arrhenius law and fragility concept. The value of generalized fragility M named here is determined and available for evaluating the stability of modified eutectic Al-12 wt%Si melts and predicting the modifying effect of P.