The association and difference between general angular momentum and orbital angular momentum are discussed. Their association lies in the identity of local property (Lie algebra) between group SU (2) and SO (3) (isomorphism), while their difference is their distinctive integral property. The minimal unit ?/2 of the general angular momentum is determined by the local properties of the groups, namely the commutation relation of Lie algebra (or commutor of angular momentum). However, the minimal unit ? of orbital angular momentum depends upon the integral property of group SO (3).