By using X-ray differaction, SEM and the measurement of sheet resistance, the effect of annealing of a-Ge/c-Ag bilayer were studied. Sheet resistance of the bilayers were measured in the range 80-300 K. The relation between sheet resistance at room temperature and annealing temperature was presented. A new diffusion mechanism was found. As the temperature increased, amorphous germanium crystallize. In the meantime, a part of Ge diffuse into Ag. When the temperature went on increasing, the germanium which diffused into silver would separate out again. The activation energy of diffusion was estimated to be 0.15 eV by means of measuring the sheet resistance of bilayer.