The development of a family of applications in a domain can be greatly eased if patterns in the domain are systematically reused. Systematic use of such patterns and their instantiation in a specific application context. In this paper, we present a prototype tool called RBML-Pattern Instantiator (RBML-PI) that generates application-specific UML class diagrams and sequence diagrams from a pattern specification describe in the Rolse-Based metamodeling Language (RBML), a pattern specification language defining a domain-specific sub-language of the UML. We give an overview of the RBML using the visitor design parttern , and demonstrate the tool using an RBML specification for CheckIn-CheckOut (CICO) domain pattern that specifies services to check in and check out items. We use the CICO pattern specification to generate an application specific UML model of a library system using RBML-PI