Experiments were conducted on bulk metallic glasses (BMGs) to identify a crucial factor that governs the degree of the plasticity shown by these alloys. It was found that in situ formation of nanocrystallites, which is known as the deformation-induced nanocrystallization, play a key role in providing initiation sites of the local deformation, indicating that a high degree of plasticity could be achieved from BMGs in which deformation facilitates nanocrystallization. The degree of this deformation-induced nanocrystallization was found to be strongly dependent on the activation energy for crystallization. In this paper, we report that a factor governing the plasticity of BMGs lies in the activation energy for crystallization.