With today?s fast changing business environment and distributed global organizations, business processes of enterprises need to be able to quickly adapt to the new business requirements. BPI middlewares are thus developed to facilitate the integration of various types of enterprise applications that operate within or across enterprise boundaries. Due to the varying complexity and distributed nature of the business processes and the wide range of enterprise applications? characteristics, capacity planning and performance tuning of BPI middleware has been a challenge for engineers who develop and deploy BPI solutions. This paper presents a layered queueing network-based performance model for a BPI middleware to address this challenge.