A method of gathering statistics of gray distribution of an image in an image processing device is provided. The method includes determining gray information of a plurality of data of an input image, wherein each piece of information corresponds to a count value. In addition, a fixed value is sequentially added to the corresponding count value according to the gray information of the data. When the count value exceeds a predetermined value, the count value is reset and the gray distribution of the image is updated. The count value of each piece of gray information is accumulated and when the accumulated value exceeds a predetermined value, the gray distribution of the image is updated to simplify the flow of process of the method of gathering statistics of gray distribution of an image.