A device of gathering statistics of the gray distribution of an image and a method thereof are provided. The device includes a comparing unit, a count unit, a memory unit, and a data allotment unit. The comparing unit determines gray information of a plurality of gray data of the input image, and output a respective count signal accordingly. The count unit includes a plurality of counters. Each counter corresponding to predetermined gray information has a count value. The counters update the count values according to the respective count signals. When the count value exceeds a predetermined value, the count unit outputs a pulse signal corresponding to the predetermined gray information. The data allotment unit updates the statistics of the gray distribution of the image stored in the memory unit according to the pulse signal. The present invention reduces the number of the registers the counters require and decreases hardware costs.