In today's distributed applications, semi automatic and semantic composition of workflows from Grid services is becoming an important challenge. We focus in this paper on how to model and compose interactively workflow applications from Grid services without considering lower level description of the Grid environment. To reach this objective, we propose a Model-Driven Approach for developing such applications based on semantic and syntactic descriptions of services available on the Grid and abstract description provided by UML activity diagram language as well. As there are particular needs for modeling composed workflows interactively from Grid services, we propose to extend the UML activity diagram notation. These extensions deal with additional information allowing an interactive and semi automatic composition of workflows. In addition this specific domain language contains appropriate data to describe matched Grid services that are useful for the execution of the obtained workflows.