Architectural patterns have an impact on both the structure and the behavior of a system at the architecture design level. However, it is challenging to model patterns' behavior in a systematic way because modeling languages do ide the appropriate abstractions and because each pattern addresses a whole solution space comprised of potentially infinite solution variants. In this paper, we advocate the use of architectural primitives for systematically modeling architectural patterns in the behavioral view. These architectural primitives are found among a number of architectural patterns and serve as the basic building blocks for modeling patterns' behavior. The main contribution of this work lies in the discovery of architectural primitives, defining architectural primitives using UML, and capturing the missing pattern semantics by using UML's stereotypes.