The invention discloses an apparatus for converting gray scale. The apparatus includes a receiving module, an encoding module, and a decoding module. The receiving module is used for receiving an input image, wherein the input image has an N-bit first gray level. The encoding module is used for selectively calculating a first color level based on both M higher bits and T lower bits of the first gray level, and then calculating a second color level and a third color level based on M higher bits of the first gray level. The decoding module is used for converting the first, second, and third color levels into an N-bit second gray level by using a gray scale conversion formula.