Prior research attempts to formalize the structure of object-oriented design patterns for a more precise specification of design patterns. It also allows automation support to be developed for user-defined design patterns in the future CASE tools. Targeting to a particular type of automation (e.g. verification of pattern instances), previous specification approaches over-specify pattern structures to a certain extend. Over-specification makes pattern specification ambiguous and disallows the specification language to be used for specifying compound patterns. In this paper, we present the structural properties of design patterns which reveal the true abstract nature of pattern structures. To support these properties so as to solve the over-specification problem, we propose an extension to UML 1.5 (basically UML 1.4 with Action semantics). The specialization and refining mechanism of UML provides also a smooth support for the instantiation, refinement and integration of pattern structures specified in UML. Our work makes no significant extension to the UML 1.5 meta-model but more in a UML Profile approach to ease the migration of our work to UML 2.0, which has not yet officially released by OMG during this work.