The effect of the free volume hole in photographic gelatin in the developing process was studied by positron annihilation spectroscopy (PAS) in this paper. A new mechanism was proposed to account for the formation of the silver filament during the development. The application of PAS introduced a new research technology and method into photographic science. The results showed that the average radius of free volume holes was maintained constant for the whole of the development. The free volume hole in gelatin restrained the reduction process of the silver ion in the spaces with different electron cloud densities. This might be one of the reasons for silver filament formation. Because of the complexity of the structure and composition of the macromolecule of photographic gelatin the free volume holes distribute in a disorderly and random manner in gelatin. It is probably the randomness and disorder that make the image silver become a loose spatial structure of silver filaments twining into balls.