Making electronic business systems, products of information technologies or solving real time tasks more and more IT specialists their designing work connect to UML language. The usage of this language provides opportunities to arrange documentation more simply, allows to communicate different position employees more easily, guarantee IT systems compatibility with business needs and requirements. One of the diagrams types used is activity diagram, which could be used to describe business processes, firms or its unit, compose official, activity or users instructions, show systems behaviour. Any investigative, designed or management activity at some cases is connected to modelling. Commonly designing systems, where parallel events could take place or, if necessary, to give an opportunity gradually, taking in account the reality, show events or state proceeding with the help of Petri nets. It is both graphical and mathematical design method. Designing activity diagrams of IT systems UML editors are used. To research and show events proceeding Petri nets are used. So the problem area is to compose a design system, which could transfer the elements of activity diagrams into Petri nets and model the functionality of the project. The major purpose of this study paper is to compose UML language activity diagrams modelling system, examine the functionality and design opportunities of system’s activity diagrams into Petri nets.