Modeling interactions among cyber-physical services is a crucial problem for analyzing behaviors of a composed system, where these cyber-physical services communicate with each other via synchronous and asynchronous messages. In this paper, we present an approach for modeling interactions of cyber-physical systems, which can be automated under the tool support. First, a service model is proposed to represent a cyber-physical service using a labeled transition system, where the order of executing transitions in a cyber-physical service is defined. Second, a synchronous composition is defined to compose a set of cyber-physical services communicating with synchronous messages into a cyber-physical system. Third, an asynchronous composition is defined to compose cyber-physical services interacting asynchronously through FIFO buffers, where three types of interactions are considered. Finally, experimental results show that our approach is automated and effective.