Abstract：Isothermal crystallization behaviors of ( Zr47 Cu44 Al9) 98. 5 Si1. 5 BMG were studied by using differential scanning calorimeter ( DSC) . Meanwhile, isothermal crystallization kinetics was analyzed by Johnson-MehlAvrami ( JMA) model. For ( Zr47 Cu44 Al9) 98. 5 Si1. 5 BMG, the Avrami exponent ( n) ranged from 1. 05 at 758 K to2. 94 at 773 K, indicating that crystallization mechanism in the alloy was governed by diffusion-controlled growth and the rate of nucleation decreases with time. With changes of n, the nucleation rate and growth pattern varied. On the basis of formula relation, the average value of activation energy was calculated to be 351 k J/mol. The apparent activation energy was strongly correlated with the fraction of isothermal crystallization.