Delegation is an important security policy that should be supported by RBAC model. The basic idea of delegation is that some active entity in a system delegates authority to another active entity to carry out some functions on behalf of the former. The grantor of the delegated roles should be responsible for the usage of them, so constraints on the usage of the delegated roles are critical components of the whole delegation model. Currently, there’re some models that extend RBAC model to support role delegation. However, their supports for constraints on the usage of delegated roles are very limited. This paper presents the requirements of role-based delegation, including temporary constraints, regular role dependency constraints, partial delegation constraints and propagation constraints. The former two kinds of constraints are modeled with a formal model – CRDM, which provides the foundation for applications in need of the constrained delegation.