A wrist structure of a robot arm includes a forearm body, a first rotating assembly, a wrist body, and a second rotating assembly. The first rotating assembly is disposed in the forearm body and drives the wrist body to rotate with respect to the forearm body in a first rotation axis direction. The second rotating assembly is disposed in the wrist body and drives a workpiece to rotate with respect to the wrist body in a second rotation axis direction. The first rotating assembly has a first speed reducer directly connected to the wrist body, so as to directly drive the wrist body at a predetermined speed reduction ratio, thereby obtaining a predetermined torque.