A robot arm assembly includes a middle joint, an upper arm rotatably connected to the middle joint, a driver rotating the upper arm mounted on the middle joint, and a transmission mechanism transferring the power of the driver to the upper arm. The driver includes an output shaft. The transmission mechanism includes a first gear coupled to the output shaft of the driver, a second gear mounted on the outer circumference of the arm, a third gear meshed with the first gear, and a fourth gear coinciding with the third gear and meshed with the second gear.