An auto-detect method for detecting a single-frame image format is provided. A single-frame image is divided into a plurality of macro-blocks. Each of the macro-blocks is divided into a plurality of sub-blocks. A meta-block is allocated in each of the sub-blocks. A pixel luminance sum characteristic value for each of the meta-blocks is calculated. A first confidence between a left half and a right half of the single-frame image is calculated according to the pixel luminance sum characteristic values. A second confidence between an upper half and a lower hap of the single-frame image is calculated according to the pixel luminance sum characteristic values. A format of the single-frame image is determined according to the pixel luminance sum characteristic values, and the first and second confidences of the single-frame image.