In this paper, the switching flow-graph models are developed to study the nonlinear behavior of DC-DC switching converters. Based on the concept of virtual switches and virtual switching functions, switching flow-graph (SFG) model are created easily without complex mathematics. The SFG modeling technique is helpful for undergraduate students or beginners to understand how the switching converters operate on different switching states. The students can also learn how to develop a model to observe the cause-effect relationship between the converter's variables. In this research, the basic Cuk converter is used as an example to reveal the dynamic behavior of the DC-DC switching converters operated in DCM or CCM. The students can further enhance their concept of circuit design by developing the SFG model in software to simulate various conditions.