In this paper, a dynamic access control mechanism with a single lock for an information protection system is proposed. Each file in the system is assigned a lock value. The access right of a user to a file is revealed by applying straightforward division operations on the file's lock. In addition, our mechanism is suitable for changing a privilege value, insertion or deletion of a user, and insertion or deletion of a file with a small change in the lock value(s). Compared to Wu and Hwangs' and Chang's methods, the computation of a lock value is simple and the verification of an access request is quite efficient.