A robot arm includes a support arm, an adjusting rod and a detecting unit. The adjusting rod rotatably extends through the support arm. The detecting unit is attached to the adjusting rod. The detecting unit includes an image capture device and a probe device. The image capture device captures images of a workpiece. The probe device includes a driving device and a probe. The driving device may drive the probe to move between a first position where the probe does not visually prevent images of the workpiece being captured by the image capture device, and a second position where the probe does block the images of the workpiece being captured by the image capture device.