The Globus Toolkit makes it very easy and comfortable for grid users to develop and deploy grid service. As for the security mechanism, however, only static authentication and coarse-grained authorization mechanism is provided in current Globus Toolkit. In this paper we address the limitations of current security mechanism in the Globus Toolkit and propose a new architecture which provides fine-grained and flexible security mechanism. To implement this without modifying existing components, we make use of the Aspect-Oriented Programming technique.