A power transmission device include a motor including a rotary shaft, a first decelerator, coupled to the rotary shaft of the motor, configured to receive power from the rotary shaft, a second decelerator, coupled to the first decelerator, configured to receive the power from the first decelerator, and a wheel bearing coupled to one side of the second decelerator. A portion of the first decelerator is configured to insert into the rotary shaft and couple thereto.