This paper describes the equivalent mechanical and electrical models for systems of the vibration suppression in flexible structures using piezoelectric elements. There are two main methods to suppress vibration of flexible structures. One is active vibration control and the other is passive vibration suppression. Because they include both mechanical and electrical systems, the mechanism of the vibration suppression is complicated and can not be understood easily. To make matters worse, the knowledge of the mechanical vibration suppression device and the electrical circuit are not utilized in those systems with piezoelectric elements. Hence, this paper describes the way to derive the equivalent mechanical and electrical models. Using those equivalent models, the mechanism of the vibration suppression can be easily comprehended and the knowledge of mechanical and electrical systems can be applied effectively.