A backlight module includes a light source, a light guide disposed on one side of the light source, a reflector disposed under the light guide, a lower diffuser disposed on the light guide, and a brightness enhancement film, which is disposed on the lower diffuser and includes a main part and several prism structures disposed side by side on a main part plane of the main part. Each prism structure includes first to third triangular prisms. The second triangular prism is adjacent to one side of the first triangular prism, and an adjacent angle adjacent to a base angle of the first triangular prism is a right angle. The third triangular prism is adjacent to another side of the first triangular prism, and an adjacent angle adjacent to another base angle of the first triangular prism is a right angle.