Software systems that have been developed over the past years have important roles in todays daily lives. When the complexity of the software system increases, it is difficult for software developers to understand the software design and with time it becomes harder to modify the software design. Today, The Unified Modeling Language (UML) is commonly used to describe the design of software systems. However, when the complexity of the software increases, the complexity of UML models increases as well. As a consequence, it becomes difficult to understand these UML diagrams. Many methods have been proposed to decrease the complexity of system software. One such method is abstraction of UML models which converts lower-level structures into higher-level structures. In this thesis, one such automated abstraction method is developed using composition reasoning and transitive reasoning. Abstraction uses transformation rules, where the pattern from the lower-level model is matched in the left-hand side, it is converted into higher-level pattern in the right-hand side. This thesis builds on an existing method abstracting class diagrams and applies the abstraction method to sequence diagrams also to demonstrate that the method is generic for other kinds of diagrams. The abstraction method is implemented in Eclipse for UML models created with the Papyrus UML editor.