A backlight module including a light guiding plate, a light source and a block is provided. The light guiding plate includes a first surface, a second surface opposite to the first surface, a light incident surface connecting to the first surface and the second surface, and a side surface opposite to the light incident surface. A thickness of at least part of the light guiding plate decreases progressively from the side surface towards the light incident surface. The light source is disposed beside the light incident surface and is capable of emitting a light beam into the light guiding plate through the light incident surface. The block is disposed beside the side surface and includes a reflecting surface facing towards the side surface. The reflecting surface is a concave surface on the block, and the reflecting surface is capable of reflecting the light beam returning to the side surface.