In these days many tools appear in market to support building system. Thus choosing the best tools need to know the most important features of it. Today UML consider as a de facto standard in software development and is used in many domains ranging from scientific modeling to business modeling. Here this paper describes the most used UML tools. It defines the main features of each one and then compares between them according to their features. It also defines the criteria for choosing the best tool in building system. Some tools aim at supporting specific life-cycle phases, but they often do not meet basic requirements arising in heterogeneous environments, UML education, early life-cycle phases, or agile processes: hassle-free tool deployment, support for fast model sketching, and flexible graphic export features.. A comparative study, including a selection of these tools, show advantages and disadvantages for each tool.