In this paper we propose a declarative method for modeling the performance impact of container middleware to component-based system. The method abstracts the major performance impact factors, and these factors are modeled as sub-models by using UML activity diagram with UML Profile for Schedulability, Performance and Time annotations. Through a model assembly descriptor file associated with specific component application, the model assembling algorithm proposed in this paper can automatically composite the performance impacting factors into component application UML model. The declarative method helps to realize the separation of concerns between component application performance modeling and middleware impact. Using a case study, we validate proposed method.