As Cloud Computing has become more and more popular, various Cloud Computing architectures or infrastructure have been defined, given their specific circumstances for the applications. However, to effectively achieve the potential of cloud computing, there is need for the definition of system architecture of the software systems involved in the delivery of cloud computing, so that it can be used as a reference for the architects or software engineering. In this paper, reference architecture of Cloud Computing is proposed. Its objective and principles are illustrated. And case studies of a SaaS, PaaS platform architecture instantiated from CCRA are given.