SHI Wen-Chang+;SUN Yu-Fang (Institute of Software;The Chinese Academy of Sciences;Beijing 100080;China) Supporting for the security policy flexibility is one of the goals of modern secure operating systems. The DTOS (distributed trusted operating system) program put forth a concept of security policy lattice, which provides a good way for the research on security policy flexibility. However, it is claimed in the DTOS program抯 description of security policy lattice that MLS (multi-level security) policies are static policies. First, an enforcement scheme for a MLS policy is constructed theoretically, which shows that MLS policies are of history sensitivity and hence have dynamic characteristics, and so that MLS policies can not be simply taken as static policies. Then, an implementation algorithm for the constructed enforcement scheme is given. It is illustrated that the constructed enforcement scheme is of the same complexity as the ordinary enforcement scheme and so is an applicable scheme. As a result, it can be affirmed that reasonable, flexible and practically feasible schemes are available to make MLS policies to be of history sensitivity. Consequently, the improperness of the assertion that MLS policies are static policies is exhibited.