A backlight module for use in a planar display panel is disclosed. The backlight module comprises a lamp tube set, a light guide plate and a positioning body. The lamp tube set comprises a lamp housing and at least a lamp tube. An optical film is disposed on the light guide plate. The light guide plate has a side for insetting the light guide plate into the lamp tube set. Furthermore, the positioning body is disposed on an inner wall of the lamp housing and comprises at least a flange against the side. The light guide plate is positioned to contact the positioning body so as to form gaps between the at least a lamp tube and the light guide plate or between the optical film and the lamp housing.