Graphical User Interface (GUI) provides a popular and convenient way for the user to freely interact with the systems which makes it widely used in various software applications, it has become an important and indispensable part of today's software. Owing to the characteristics of GUIs different from the traditional software, traditional test techniques and methods cannot satisfy the requirements of GUI testing. Modeling and testing of GUIs-based system is a difficult and challenging work. GUIs-based application is an event-driven application. In GUIs, there exist not only the input events and output events, but also the internal events. In this paper, we identify the input events, output events and internal events and propose an approach to modeling and testing of GUIs-based system using the IOLTS, and input events, output events and internal events are also taken into account. Constraints on events and regular expressions on validation of data are given out. The interactions of GUIs are constructed by the corresponding output events. Finally, tests generation and tests instantiation are given out.