UML activity diagrams and sequence diagrams describe the behavior of a target domain or a system from different viewpoints. When we use these diagrams for modeling the same matter in an application, these diagrams, or the models written by them, must be consistent from each other. However, the evaluation for the consistency is difficult, since these diagrams have considerably different syntax and semantics. This paper presents a process algebraic approach to evaluating the consistency between these models. CCS (Communicating Sequential Processes) is used as process algebra.