A backlight module (1) has a light guiding device (2) and a light source (200). The light guiding device has a single light guide plate (20), and a reflection member (26). The single light guide plate has a light incidence surface (211), a top light emitting surface (23), and an opposite bottom light emitting surface (22). The single light guide plate further has a plate cavity (not labeled) formed therein for receiving the reflection member. The reflection member has a top reflection surface (28) and an opposite bottom reflection surface (29). The top reflection surface of the reflection member faces toward the top light emitting surface of the light guide plate, and the bottom reflection surface of the reflection member faces toward the bottom light emitting surface of the light guide plate.