The several factors on which the outer quantum efficiency of the silicon photodiodes depend upon, especially the collection efficiency of the devices have been discussed. The universal formula and simplified formula for determining "the quan-tum feffioienoy of the devices have also been shown. For three silicon phoiodiodes on which ihe self-calibration have been done we obtained ihai -the unoeriainij of determining outer quantum efficiency was about ±0.05%, ihe consistency of measuring laser beam power was abou-t ±0.05% also, and finally the agreement with the results from the absolute radiometers was better than ±0.2%.