Reuse-based software development provides an opportunity for better quality and increased productivity in the software products. One of the most critical aspects of the quality of a software system is its performance. The systematic application of software performance engineering techniques throughout the development process can help to identify design alternatives that preserve desirable qualities such as extensibility and reusability while meeting performance objectives. In the present scenario, most of the performance failures are due to a lack of consideration of performance issues early in the development process, especially in the design phase. These performance failures results in damaged customer relations, lost productivity for users, cost overruns due to tuning or redesign, and missed market windows. In this paper, we propose UML based Performance Models for design assessment in a reuse based software development scenario.