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Cyber-Physical Systems (CPSs) integrate computing, communication and control processes. Therefore, both time and space information are required in the design and modeling of CPS systems. CPS also requires demanding real time property. However, unified modeling of continuous and discrete variables is still an unsolved problem in the CPS development. In this paper we propose an extended UML state chart, Spatio-Temporal UML state chart for Cyber-Physical Systems (STUML State chart). This state chart is based on the UML Profile for Modeling and Analysis of Real-time and Embedded (MARTE) systems. In STUML State charts, we unify the logical time and the chronometric time variables, and extend the traditional events to CPS events. Based on MARTE and hybrid automata, we improve the STUML state chart, and based on Labeled Transition Systems, we define formal syntax and semantics of STUML state charts. At the end of this paper, an Intelligent Transportation System is employed as an example to show how to model the system behavior with STUML state charts.