Abstract: Only taking into account the results measured in Stern -Gerlach experiment，we obtain that the commutator relations of spin operators are similar to the ones of orbital angular momentum operators by using the rule of taking trace of matrix without introducing the matrix representation of spin operator. Then we conclude that the spin and orbital angular momentum belong to the same kind: angular momentum，which is different from the original concept of spin introduced in history and the one in quantum mechanics textbooks. We attempt to discuss about that the spin belongs to angular momentum with the philosophy“birds of a feather flock together”.

Key words: Stern-Gerlach experiment, spin, trace of matrix, commutator, operator