The missions of complex systems, organizations, or groups can be identified through careful requirements and domain analysis. Mission architecture modeling is a crucial step for enterprise modeling and design. However, the concept of mission modeling is absent from the Unified Architecture Framework (UAF), in which the system engineers have to specify and model from the sketch. In this paper, we propose a Mission Architecture Modeling Language (MissionML), a two-layer architecture language that generalizes the general common knowledge and special knowledge from five typical missions as a shared layer and specific characteristic layer. Moreover, MissionML is implemented as a UAF profile, incorporating numerous domain concepts in its syntax and semantics for mission modeling. Finally, we use five public case studies to demonstrate the learnability and extensibility from the view of system engineers.