A power supply apparatus is electrically connected with a load and one of a second substrate, a third substrate and a fourth substrate each of which has a first locking portion. The power supply apparatus comprises a first substrate, a power distribution circuit and a position board. The first substrate comprises a first coupling portion and a second coupling portion. The power distribution circuit is disposed at the first substrate, receives the external electric power and converts it into a supplying power. The position board comprises a second locking portion that is coupled to the second coupling portion of the first substrate when the power supply apparatus is electrically connected with one of the second substrate and the third substrate. The first locking portion of one of the second substrate, the third substrate and the fourth substrate is coupled to the first coupling portion of the first substrate.