Abstract: The Unified Modeling Language（UML）, published by Rational Software Corporation and other UML partners, is attracting wide attention in the area of object technology. The historical background and the main contents of UML are introduced and then its significance, positive influence and some possible problems are discussed. UML is an expressive and powerful modeling language; however, it cannot be concluded that it would replace all existing object oriented analysis and design methods. The reason is that UML is only a modeling language rather than a method, and its excessive complexity might become an obstacle to win a great number of users.