This paper gives firstly an introduction about the consistency of an Access Control List (ACL). It uses triples or in other words, in three dimensions and their corresponding decisions to describe the access control rules, i.e. ACL entries. We present then and justify the mathematical definitions of an ACL and its related components and discuss its consistency, two theorems about the consistency of the ACL. Thirdly the paper discusses verification of the consistency of the ACL and a new method used to verify the consistency of the ACL derived from the theorems in use. The method obtained here is a theoretical approach. In the conclusion, we point out that in a practical system, verification of the consistency of the ACL is necessary and the method is an effective and efficient one, further measures should be taken and study should be continued to reduce the amount of computation needed to examine the consistency of the ACL.