Cloud Computing is a model of both service delivery and consumption for increasing the capabilities of enterprises dynamically without investing in new infrastructure, training new personnel, or licensing new software. It refers to everything delivered as services over the Internet. Although the benefits of Cloud Computing are evident, security is one of the major issues which hamper the growth of cloud. In this paper, to facilitate identifying the security threats of cloud computing, we Taxonomy the security aspects of cloud computing systems and summarize the main security threats of it. Our taxonomy covers the four domains of cloud computing systems: infrastructure layer, platform layer, application layer and administration.