In the past the most of the traditional PACS adopt two-tier Client/Server architecture based on DICOM network in china. However, with the developments of PACS that becomes more large-scale and distributed, the traditional PACS architecture has a lot of weaknesses. To overcome these shortcomings, this paper presents a CORBA-based PACS in three-tier architecture which consists of the application-tier, the middle-tier and the DICOM data-tier. In application-tier, the upper applications can use the service offered from the middle-tier through the application interface. In middle-tier, the characteristic service of PACS, which are extracted from the traditional PACS, are wrapped in service components plug in the ORB software bus. For the data exchange in CORBA-based implementation, the middleware uses IIOP to call service components through the ORB then to return the results to the application-tier. The CORBA object services are responsible for the object location, creation, implementation and destruction, etc. In DICOM data-tier, DICOM image modalities and DICOM databases provide DICOM data for the middle-tier through DICOM image access service. Furthermore, a design of DICOM image access service, which accesses DICOM data and shields the middle-tier from the complexities of DICOM data structure and DIMSE protocol, is presented in this paper. After referred to the CIAS (Clinical Image Access Service) specification defined by OMG, a simplified IDL interface of DICOM image access service is also described. The CORBA-based PACS architecture is an open architecture allowing for the scalability and the interconnectivity & interoperability.