Testing is an important task in the development of systems and a task which gets more difficult as the scale of the system under test (SUT) increases. This thesis focuses on conformance testing in the telecommunication field which ascertains that the implementation of a communication protocol adheres to its requirements. Model based testing is an emerging way of testing, which generates lengthy and complicated test cases systemically from the more compact model of SUT. A model of SUT represents the data structures used by and the behavior of SUT on an abstract level, and it can be described in various ways, for example, in a script language such as TTCN-3 or in graphical format such as a state machine graph. This bachelor’s thesis deals with the design and implementation of a tool where the users can design the data structure of the model of SUT on a graphical user interface and TTCN-3 code describing the data structure of the model is generated from the visual representation of the model. The development of a tool where the user can draw the behavior of the SUT as well as data structures with a graph is expected as a future work of this task. Once completed, this will be a powerful test modeling tool for test engineers familiar with TTCN-3.