A one-piece cap comprising the cap made by a hot press forming method, including a bowl-shaped cap body and a cap visor connected with the cap body; the method of making the one-piece cap comprising steps of preheating a hot pressing die, preheating a hot pressing die, pressing and shaping materials, cutting off the excessive material, and trimming for a bound edge; the one-piece cap being made without sewing or bonding the cap body and the cap visor separately in the manufacturing process, and therefore, reducing the production procedures and costs, and saving the production time and the manual labor, thus the one-piece cap has simple structure and beautiful appearance with more round and smooth lines.