A robot arm includes a working arm member, a workpiece placement unit, and a driving device. The workpiece placement unit includes a base seat coupled to the working arm member, and a hollow supporting rod with a suction unit. The supporting rod extends from the base seat, is adapted for supporting a workpiece thereon, and defines a passage therethrough. The supporting rod is made of a composite material including a hollow rod structure having at least one fibrous layer that is made of resin-impregnated carbon prepreg, and an outer covering layer made of fiberglass material and covering an outer surface of the hollow rod structure. The driving device is coupled to the working arm member, is operable to drive the working arm member for moving the workpiece placement unit, and is further operable for extracting air from the passage so that the workpiece supported by the supporting rod is held in position by the suction unit.