A Novel Substitution Judgment Method for Mobile Cloud Computing Application System Components Mobile cloud computing is a novel computing model, which integrates the cloud computing into the mobile environment and overcomes obstacles related to the performance, environment and security discussed in mobile computing. For a mobile cloud computing application system (MCCS), reliable components substitution is an important issue for troubleshooting and system upgrading. In this paper, this issue is equated to MCCS services substitution based on a service-oriented architecture of MCCS. Further, a formal method for modeling MCCS service is proposed based on space-time Pi-calculus, which is presented through introducing location and time operators into classical Pi-calculus. Then, a decision theorem for MCCS service substitution is put forward according to the relationship between service compatibility and substitution. Finally, a case study is performed to show that how to apply this method to ensure MCCS components substitution. The experiment result shows that it is reasonable and feasible.