Abstract: Aiming at the difficulty in determining the mapping range in gray-level transformation,a novel image enhancement method based on mapping threshold is proposed. The lowest point between target area and background area is searched automatically in the mapping transformation of histogram gray-leve1. Then it is operated as a threshold for mapping of image gray-level scale so as to adjust the gray coverage of the image. Finally,the brightness and contrast of the image are improved. Experiment analysis demonstrates that, the proposed method can adjust the image dynamic gray-level range so that obtain better enhancement effects than conventional gray histogram transformation. The new method has solved the problem of determining the image gray-level scale in the mapping transformation,is an effective image enhancement method.