Service composition is considered an efficient way to complete complex task in mobile ad hoc networks. Most previous works focusing on composite service execution are based on the centralized architecture while the decentralized architecture can avoid the large amount of round-trip messages thus significantly reduces the transmitting costs in the network. However, to implement the decentralized service execution architecture, the existing Web service choreography languages are not adaptable. In this paper, we first present the decentralized execution architecture for composite service with the analysis of its advantages. Then, we propose an extension for the service choreography language to support the decentralized service execution architecture. The prototype system is presented to show the practicability of our approach.