The pressure-induced phase transition in amorphous silicon (a-Si) is studied using ab initio constant-pressure molecular-dynamic simulations. Crystalline silicon (c-Si) shows a phase transformation from diamond-to-simple hexagonal at. 29.5 GPa, whereas a-Si presents an irreversible sharp transition to an amorphous metallic phase at 16.25 GPa. The transition pressure of a-Si is also calculated from the Gibbs free energy curves and it is found that the transformation takes place at about 9 GPa in good agreement with the experimental result of 10 GPa. We also study the electronic character of the pressure-induced insulator to metal transition.