This paper studies Portevin-Le Chatelier (PLC) effect as a form of dynamic material instability. In such phenomenon negative rate dependence is coupled with the appearance of a selfsustained oscillatory behavior in solid bodies. The tools of the investigation are of the theory of dynamical system. By selecting a proper form of the constitutive equation of the rate dependent material, a dynamical system will be constructed from the basic equations of continua. Then the birth of a self-sustained oscillation is detected at the conditions of a dynamic material instability mode. This results show an interpretation of PLC as a flutter type of loss of material stability.