This invention is related to a lightweight process of manufacturing a bicycle's rim and a rim structure manufactured thereby. A rim frame comprising an inner frame, a cladding part, and a support part between the inner frame and the cladding part. The inner frame and the support part are milled to form plural through holes uniformly distributed thereon. The areas occupied by the through holes is 35% to 85% of a total surface of the inner frame and the support part. Moreover, a proper distance is formed between the adjacent through holes in the process, and a carbon fiber layer covers the outer surface of the rim frame. Therefore, the weight of the bicycle's rim is decreased, and the carbon fiber layer can stably attach on the rim frame to increase the structural strength of the rim.