In system testing the system under test (SUT) is tested against high-level requirements which are captured at early phases of the development process. Logical test cases developed from these requirements must be translated to executable test cases by augmenting them with implementation details. If manually done these activities are error-prone and tedious. In this paper we introduce a model-based approach for system testing where we generate first logical test cases from use case diagrams which are partially formalized by visual contracts, and then we transform these to executable test cases using model transformation. We derive model transformation rules from the design decisions of developers.