In today’s environment, the day to day business operations of organisations heavily rely on the automated business processes from enterprise IT infrastructures. The dynamic business environment and the problems with the long time implementation, high cost, etc. of process development and maintenance, are pushing organisations as process consumers to look for ready to use and shared business processes from IT providers for on demand requirements. This is manifest in the rising of Cloud Computing and Business Process Outsourcing with the development of the new concept of (business) Process as a Service. Service-Oriented Architecture (SOA) is an architectural style commonly adopted for enterprise IT infrastructures and the implementation of service based business processes. However, the SOA style and current specifications do not intend for the case of business processes sharing with cross organisational consumers, since various requirements or the business policies of different organisations, are unmanageable to meet on a business process at the same time. In this thesis, we present an architectural solution to address the above issues for the Process as a Service. It consists of a Service Process Architecture (SPA) architectural style designed to extend the SOA style, and a supported architecture framework designed for the specific style. The proposed SPA style has a defined principle for the goal of process customizability and adaptability on process design and development with providers. The supported architecture framework consists of three main parts: a policy specification entails expressing business policies or the requirements of consumers regarding business processes in the cloud; a coordination framework aim to enforce expressed policies on process executions with adaptive business processes for different consumers; finally, an AOP enhanced extension is responsible for the extensibility of the framework to satisfy consumers’ possible additional requirements. Our SPA style could extend the SOA style, and has an impact on the SOA principles. With the supported architecture framework, we provided a complete architectural solution for the Process as a Service.