We have studied the lithiation-induced amorphization of crystalline silicon by using first-principles calculations. Ab initio simulations show that crystalline silicon amorphizes at a ratio of 0.3 Li atoms per Si atom. Upon amorphization, the tetrahedral Si network suddenly collapses and the average number of Si atoms around Li significantly decreases from 4.6 to 2.6. We suggest the presence of ring structures, made of pure Si atoms and composite Li–Si atoms, as representative local structures of Li–Si amorphous alloy.