A light mixer and a backlight module having the light mixer are provided. The backlight module includes a plurality of light mixers and a diffuser. The light mixers are arranged on a plane in an array. Each light mixer includes a light mixing plate, a plurality of light sources, a plurality of upper reflecting layers, a lower reflecting layer, and a semi-permeable layer. The light mixing plate has an upper surface, a lower surface, a peripheral surface, and a plurality of receiving holes. The light sources are respectively located in the receiving holes. The upper reflecting layers are respectively located right above the light sources. The lower reflecting layer is located on the lower surface of the light mixing plate. The semi-permeable layer wraps the peripheral surface of the light mixing plate. The diffuser is located above the light mixers. Therefore, the fabricating process of the backlight module is relatively simple, the fabricating cost is relatively low, and it is simple and convenient to maintain. Furthermore, the special design of the light mixers enhances the light mixing efficiency and the luminance.