A backlight module includes a lamp set, a light guiding plate, a diffuser, a top prism sheet, and a down prism sheet. The diffuser includes a first end and a second end opposite to each other, the first end of the diffuser is closer to the lamp set than the second end, and the diffuser includes a first end and a second end opposite to each other. The diffuser includes the overlapped portion stacked with the top/down prism sheets and the extended portion extending from the overlapped portion. On the first end of the diffuser, the light beams emitted by the light guiding plate may be processed by the first extended portion, instead of being emitted out directly. This can avoid the bright lines occurred by the backlit so as to reduce the optical leakage. In addition, a display device is also disclosed.