A backlight module includes a tray, a light guide plate, a lighting device, and an optical film set. The tray includes a bottom plate and a side wall connecting an edge of the bottom plate. The side wall includes a plurality of wall portions connected to one another. Some of the wall portions are perpendicular to the bottom plate, and the others are parallel to the bottom plate. The light guide plate is disposed on the bottom plate. The lighting device is disposed at a light incident side of the light guide plate. The optical film set disposed on the light guide plate leans against the side wall.