The present invention provides a light guide plate and a backlight module. The backlight module comprises the light guide plate. The light guide plate comprises a plurality of elongated microstructures disposed on the light guide plate and parallel to a light-incident surface thereof. At least one curve parameter of profiles of the elongated microstructures is altered according to a position variation in an extending direction of the elongated microstructures, so as to adjust a brightness distribution in the extending direction of the elongated microstructures. The invention can improve a luminous efficiency of the light guide plate.