Abstract: As mechatronics software systems become larger and more complex, the need to ensure system consistency and completeness with respect to the requirements and to manage system complexity becomes more important. From a system point of view, complexity depends on the number and the multiplicity of system constituent units and their relationships. In this paper, the initial results from our efforts to adopt the concept of software architecture in the development of mechatronics software systems for the purpose of managing complexity and ensuring system qualities are presented. A design framework for architecting mechatronics software systems is being developed. The design framework includes two models: (1) an architecture model, and (2) a decision model. The architecture model aims to provide an effective basis for managing complexity and performing architecture based system development. The decision model aims to provide a basis for reasoning about how to meet flexibility-related qualities in the development.