A polish head of a chemical mechanical polishing system is provided. The polish head includes a carrier head, a membrane mounted to the carrier head, an inner retaining ring mounted to the carrier head and surrounding the membrane, an outer retaining ring mounted to the carrier head and surrounding the inner retaining ring, and an image capturing device. The outer retaining ring is spaced apart from the inner retaining ring. The image capturing device is mounted to the carrier head and between the inner retaining ring and the outer retaining ring.