A light guide device (60) and a backlight module using the same. The light guide device includes: an incident surface (61); an emitting surface (64) adjacent to the incident surface; a reflecting surface (62) opposited to the emitting surface; a plurality of first V-shaped structures (644) formed on the emitting surface; and a plurality of second V-shaped structures (622) formed on the reflecting surface and perpendicular to the first V-shaped structures, wherein respective heigths of the second V-shaped structures increase with increasing distance from the incident surface and respective distances between adjacent second V-shaped structures decrease with increasing distance from the incident surface. The light guide device of the present invention does not need optical elements, and thus has a simple structure. A backlight module using the same light guide plate is also provided.