An in-wheel assembly is provided, and the in-wheel assembly includes an in-wheel actuator and a wheel. The in-wheel actuator includes a driving motor; a decelerator which is disposed inside the driving motor and configured to reduce a rotational speed of the driving motor; and a hollow shaft which is disposed inside the driving motor and configured to transfer a rotational force of the driving motor to the decelerator, which is accommodated in a hollow of the hollow shaft, and circulates fluid inside of the driving motor with respect to a surface of the decelerator when driven to rotate by the driving motor. The wheel accommodates the in-wheel actuator and receives a rotational speed reduced by the decelerator to rotate.