An alignment tool for assembling a rear plate and a frame of a flat display panel is introduced. The rear plate includes a front surface, a rear surface opposing to the front surface for adhering with the frame in an aligning process, and an alignment mark located at the front surface. The alignment mark has a predetermined thickness and is protruded from the front surface. The alignment tool includes a base, a clamp arm and an upper arm. The base further includes a base top surface and an alignment notch. The base top surface is used for contacting with the front surface of the rear plate and the alignment notch has at least two alignment sides for matching with the alignment mark. The clamp arm is protruded from the base, and the upper arm is protruded from the clamp arm and used for aligning with the frame. The upper arm includes an arm lower surface parallel to and spaced apart from the base top surface by a predetermined distance for forming an open space in between. The rear plate is sandwiched in the open space during the aligning process, and the alignment mark is leaned upon the alignment sides of the alignment notch, such that the alignment mark is inserted into the alignment notch.