Amorphous alloys containing nanoscale crystals have been deformedin situin a high-voltage electron microscope. Acceleration in the nucleation and growth of nanoscale crystals has been found at a crack tip during ageing under stress. It has also been revealed that the nanoscale crystals are free from lattice defects after deformation to cracking. These observations suggest that the presence of non-deformable nanoscale crystals plays an important role in the strengthening of amorphous alloys.