The present invention discloses a backlight module which can be incorporated within the liquid crystal display device, and which includes a waveguide, a heat dissipating frame, a light source and a backframe. The heat dissipating frame includes a carrying portion and a sidewalk. The carrying portion abuts against a bottom surface of the waveguide. The carrying portion is defined with openings in an area adjacent to the sidewalk. The light source is mounted onto the sidewall and offset to the opening. The backframe includes a bottom board and erections which extend and pass through the opening. The bottom board abuts the carrying portion. The thickness of the erection is larger than the thickness of the light source such that expansion of the waveguide exposed under heat will not in contacting with the light source. Accordingly, the light source will not be damaged by the expansion of the waveguide, and therefore prolong the service of the light source.