Abstract: Robust control theory is useful when the plant may be stable, unstable, marginally stable, non-minimum phase or even certain extend nonlinear systems. In this study a ball and beam system is considered as an example with two poles at origin. Which is considered as unstable system. It is a special class of unstable system. A robust controller is designed to satisfy the robust performance criterion and the designed controller is compared with classical PID controller design. Also, the internal stability is checked by looking into the zeros of the characteristics equation.