A backlight module having a light guide plate with prismatic structures and a manufacturing method thereof are provided. The backlight module includes a light guide plate, a brightness enhancement film, and a light source module. The light guide plate includes a first surface and a second surface opposite to each other. The first surface has prismatic structures disposed side by side thereon. Each of the prismatic structures includes a first slope and a second slope being adjacent to each other and having an included angle with respect to the first surface, respectively. The included angles are equivalent. The brightness enhancement film includes a third surface and a fourth surface opposite to each other. The third surface faces the second surface. The fourth surface has prisms disposed side by side thereon along the extending direction of the prismatic structure. The brightness enhancement film has an optimum incident angle associated with an angle of the prism and corresponding to the included angles. The light source module is disposed beside the light guide plate along the extending direction of the prismatic structure.