There are many information objects and users in a large company. It is important issue how to control users access in order that only authorized user can access information objects, Traditional access control models do not properly reflect the characteristics of enterprise environment. This paper proposes an improved access control model for enterprise environment. The characteristics of access control in an enterprise are examined and a task role-based access control(T-RBAC) model founded on concept of classification of tasks is introduced. T-RBAC deals with each task differently according to its class, and supports task level access control and supervision role hierarchy.