A wrist housing, a wrist rotatably connected to the wrist housing, a first driver, a first transmission mechanism, a rotary member, a second driver, and a second transmission mechanism makes a robot arm assembly. The wrist housing is hollow. The first driver is assembled within the wrist housing for driving the wrist to rotate relative to the wrist housing along a first rotary axis. The first transmission mechanism is also assembled within the wrist housing and is positioned between the wrist and the first driver. The rotary member is rotatably assembled to a distal end of the wrist along a second rotary axis. The second driver is assembled within the wrist housing for driving the rotary member to rotate. The second transmission mechanism is assembled within the wrist housing, and is positioned between the second driver and the rotary member.