A holder is disposed at a side of a back light module adjacent to a light incident side of a light guide plate (LGP). A light source module is fixed on a wall of the holder that faces the light incident side. A holder engager on a side wall of the LGP engages with an LGP retainer of the holder. When the LGP expands or contracts due to changes of temperature, the holder and the light source module move with the side of the LGP, thereby a gap between the light source module and the light incident side is maintained and the luminance of the backlight module is stabilized.