Dependence of the sensitivity to grain diameter on ultra-fine-grain (UFG) emulsions was investigated. Sensitivity decreased in proportion to fourth to fifth power of the grain diameter. The sensitivity decline factor peculiar to the UFG emulsion exists. Large increase in sensitivity appeared on a low intensity exposure by the sodium nitrite treatment, and it was also remarkable with the UFG emulsion. Sensitivity was in proportion to the third power of the grain diameter at this time. It got rid of the sensitivity decline factors on the UFG emulsion by the sodium nitrite treatment. Because a sodium nitrite acts as a halogen acceptor, the sensitivity decline factor is the rehalogenation. It can think that the rehalogenation becomes easy to happen on the UFG emulsion because silver atoms and halogen atoms would be formed nearby.