A light guide plate and a backlight device are provided. The backlight device may include a guide plate include a first light guide having a first inclined plane and a second light guide having a second inclined plane facing the first inclined plane. A refractive index of the second light guide may be different from that of the first light guide. An optical device may be provided on the light guide plate. A first light emitting module may be provided at a lateral surface of the first light guide and a second light emitting module may be provided at a lateral surface of the second light guide. A reflective device may be provided below the second light guide.