Cloud computing is a new paradigm which enables users to reduce their costs and is advantageous to both the serving and served organizations. However, security issue is a major concern in the adoption of cloud computing. The most effective way of protecting cloud computing services, resources and users is access control. This paper intends to provide a trust-based access control mechanism for cloud computing considering its multi-domain aspects. Trust is introduced into cloud computing environment and trust relationships between users and cloud platform are built.