A liquid crystal display (LCD) and a back light module are provided. The back light module includes a light guide plate (LGP) and a plurality of light-emitting devices. The LGP has a plurality of partitioning grooves, a bottom surface, a light exiting surface, a first side and a second side. The bottom surface is opposite to the light exiting surface. The first side and the second side are located at the two opposite side surfaces of the LGP. The extension direction of each of the partitioning grooves extends from the first side towards the second side. Each of the partitioning grooves has an opening and the opening is located at the bottom surface. The plurality of light-emitting devices are disposed at least one of the first side and the second side. The liquid crystal panel of the LCD is disposed at the light exiting surface.