In this paper we discuss a requirement engineering process how to generate a user interface prototype from scenarios and yield specification of system in form of Colored Petri net. Thereafter scenarios are described in form of sequence diagrams defined by the Unified Modeling Language (UML)with additional user interface information. These diagrams are transformed into Petri net specifications which can capture the behavior of the entire system. From global specification , a user interface prototype is generated and embedded in a user interface builder environment for further refinement . With end user's feedback and verification, the user interface prototype may be iteratively refined.