A robot joint utilized in a robot arm is disclosed. The robot joint includes a hollow shaft, a first gear, plural motors, plural second gears, an encoder, and a digital signal processor. The first gear is fixed on the hollow shaft. The motors are arranged surrounding the hollow shaft. Each of the motors has a rotating shaft. The second gears are fixed on the rotating shaft and are engaged with the first gear, so that the hollow shaft can be rotated with the second gears driven by the motors. The encoder is disposed at a side of one of the motors opposite to the second gears. The signal detected by the encoder is sent to the digital signal processor for driving the motors. A robot arm using the robot joint is also disclosed.