A power transmission mechanism and a robot arm using the same are described. The power transmission mechanism at least includes a first power source, a second power source, a plurality of steel ropes, a support arm component, a stopping component and a joint component. The stopping component is disposed in the support arm component and includes a first stopping unit and a second stopping unit that rotate coaxially. The first and second power sources and the joint component are disposed at two ends of the support arm component. The first and second power sources transmit power via the first stopping unit and the second stopping unit through the plurality of steel ropes to drive the joint component to perform a two-degree-of-freedom motion. The support arm component and the joint component can be applied in an upper arm and an elbow of a robot arm, thereby constituting a multi-degree-of-freedom robot arm.