The performance of two methods for designing robust positioning control systems for flexible robot arms using two-degrees-of-freedom controllers are compared through experiments. Both methods design the feedback controllers based on H∞ control theory by solving two algebraic Riccati equations. The first method designs the feedforward in frequency domain based on model matching method using the inverse dynamics of the arm system. The second designs the feedback part in time domain using the inverse dynamics of the non-minimum phase system of the arm. Results showed that the second method performs better tracking than the first method and provides an alternative design method of a robust positioning control system of flexible arms.