A robot arm assembly includes a support arm, a lower arm rotatably connected to the support arm, an upper arm rotatably connected to the lower arm, a first driver for driving the lower arm, a first transmission transferring power from the first driver to the lower arm, a second driver for driving the upper arm, and a second transmission transferring power from the second driver to the upper arm. The first and second drivers are mounted on the support arm. The second transmission includes a belt assembly, a beveled gear assembly driven by the belt assembly, and a speed reducer coupled to the upper arm.