This paper reports the study of the effect of free drying dehydration on diffraction efficiencies of dichromated gelatin holograms of different gelatins. It was found that the drying can greatly increase diffraction efficiencies of dichromated gelatin holograms, ranking from 30～45% to 55～80% for dichromated gelatin holograms of inert gelatins and from 75～90% to 90～100% for dichromated gelatin holograms of active and PA modified gelatins after drying. But at the same level of exposure, the diffraction efficiencies of dichromated gelatin holograms of inert gelatins generally increased more than those of active gelatins; However, for the same gelatin, the low diffraction efficiencies dichromated gelatin holograms of underexposure and overexposure increased more than the high diffraction efficiencies dichromated gelatin holograms of moderate expoosure. The reason why the diffraction efficiencies are changed by drying was discussed.