To ensure the reliability of UML2.0 sequence diagrams (SD), which are used in software analysis and design, propositional projection temporal logic (PPTL) model checking is adopted in this paper. First, event deterministic finite automata (ETDFA) are proposed and used to describe the formal models of SD. Furthermore, an algorithm for model checking ETDFA with PPTL formulas being the properties is presented. Finally, based on the implementation of PPTL model checker, the ETDFA models of SD are verified. Experimental results show that the proposed method is useful in ensuring the reliability of SD.