In SOA, service composition and selection can be viewed as delegation behaviors. Due to multi-levels service workflow, the delegation relationship among services is often confused and the overall trust of component services by the requester cannot be evaluated quantitatively. In this paper, we propose a model to depict the delegation relationship and quantitatively measure the implied trust degree among services in SOA. Firstly, we use Hierarchical Elementary Net System (HENS) that is based on the Petri Net to model the service workflow. Then, a Delegation Tree is abstracted from the HENS. Based on the Delegation Tree, we can derive the delegation relationship among services, calculate the trust degree among services which have delegation relationship, and find out the Critical Trust Path in the Delegation Tree. This model is important because it can quantify service delegation in SOA; it can also be used to guide delegation process thus reducing service risk due to delegation.