Amorphous materials are used for both structural and functional applications(1-5). An amorphous solid usually forms under driven conditions such as melt quenching(4), irradiation(6), shock loading(7-9) or severe mechanical deformation(10). Such extreme conditions impose significant challenges on the direct observation of the amorphization process. Various experimental techniques have been used to detect how the amorphous phases form, including synchrotron X-ray diffraction(11), transmission electron microscopy (TEM)(12) and Raman spectroscopy(13), but a dynamic, atomistic characterization has remained elusive. Here, by using in situ high-resolution TEM (HRTEM), we show the dynamic amorphization process in silicon nanocrystals during mechanical straining on the atomic scale. We find that shear-driven amorphization occurs in a dominant shear band starting with the diamond-cubic (dc) to diamond-hexagonal (dh) phase transition and then proceeds by dislocation nucleation and accumulation in the newly formed dh-Si phase. This process leads to the formation of an amorphous Si (a-Si) band, embedded with dh-Si nanodomains. The amorphization of dc-Si via an intermediate dh-Si phase is a previously unknown pathway of solid-state amorphization.