This paper describes process of mobile car sharing application development i.e. a product designed for users of car sharing systems. If there is a need for such product, similar products are compared and target users are interviewed. After defining user demands, functional flow diagram, Express-G diagram and UML diagram are created to define basic data entities and attributes, to visualize system architecture, build elements and their interaction. System is designed as a part of Smart City initiative. The goal of Smart City initiative is to improve citizens’ life quality through use of systems that improve economy and save the environment.