The invention provides a Gamma conversion system applied on a source of image pixel data. Each image pixel data in the source of image pixel data has an M-bit pixel value with a first Gamma exponent. The Gamma conversion system comprises a storage module, a processing module, and a selecting module. The storage module therein stores I look-up tables. The processing module is respectively coupled to the source of image pixel data and the storage module, and the processing module comprises I converting modules, each of the converting modules corresponds to one look-up table. The selecting module is coupled to the source of image pixel data and the processing module respectively, and generates a selection signal according to the M-bit pixel value. In response to the selection signal, the processing module assigns one of the converting modules to convert the M-bit pixel value into an N-bit pixel value with a second Gamma exponent according to the look-up table corresponding to the assigned converting module.